DRY LAID PROCESS - CARDED
The carded process uses rotating cylinders covered with fine wires with teeth that comb the fibers into parallel arrays. The web properties are anisotropic (stronger in the direction in which the machine makes the fabric than in the direction across the machine). Some cards have randomizing sections that change the direction of fibers as they are laid down in order to produce fabrics with increased cross directional strength. Synthetic and natural fibers up to 150 millimeters long can be used. The weights and thicknesses of carded webs can be increased many-fold by overlapping layers of webs or by pleating a single web. The largest carded nonwoven market in the world is cover stock for absorbent products. Most cover stock is carded thermally point bonded polypropylene fabric which competes with spunbonded polypropylene nonwovens in cover stock applications. Apparel interlinings are the next largest carded applications, followed by consumer and industrial wipers, fabric softener substrates and filtration media.

DRY LAID PROCESS - AIR LAID
Air laying suspends the fibers in air, then collects them as a batt on a screen that separates the fibers from the air. It is restricted to relatively short fibers and the fiber orientation in the web is generally random. Several manufacturing systems that handle longer fibers are especially useful in preparing thick webs containing randomly laid fibers. The heavier weights are attained by slowing down the fiber conveyors and increasing fiber feed throughput. All air laid systems require machines that separate the fibers and then deposit them onto rotating perforated cylinders or moving screen belts. Some systems use a card to further separate the fibers before they are deposited in a final web form. The depositing is done with the aid of an air stream. Virtually all of the volume of air laid fabrics is used in disposable wiping products. Industrial dry wipers and premoistened consumer wipes are the largest end-uses. These fabrics are also used as the absorbent cores for sanitary napkins and underpad products. They are also used for a range of medical and other industrial applications which require lightly absorbent materials.

WET LAID
In a wet laid process, fibers are suspended uniformly in water, at very high dilutions of .001 to .005% weight of fiber, and collected to form a sheet by separating the fibers from the water onto a screen - as in papermaking. The screen can be in the form of a wire belt in an inclined position, or a cylinder, where the fiber suspension is fed onto the screen; or two wire belts where it is fed between the two belts. The wet web is squeezed between rolls to remove most of the water and dried further by passing it through ovens. The fibers are laid down in a random orientation to one another. The final fabric has relatively isotropic properties (properties such as strength are similar in all directions in the plane of the fabric). The wet laid process permits adding binders, chemicals and colorants before or after the web is formed but is generally restricted to very short fibers, in the range of 2-6 millimeters, in most commercial processes. Spunbonded and melt blown extrusion processes make webs directly from filaments as they are being spun from molten polymer resin.

WET LAID - SPUNBONDED
Spunbonded nonwovens are made in a process in which a thermoplastic fiber forming polymer is extruded through a linear or circular spinnerette. The extruded polymer streams are rapidly cooled and attenuated by air and/or mechanical drafting rollers to form desired diameter filaments. The filaments are then laid down onto a conveyor belt to form a web which is bonded to form a spunbonded web. Spunbonding is an integrated one step process which begins with a polymer resin and ends with a finished fabric. A version of the spunbonded technology is flashspinning. In this process, high density polyethylene is dissolved, extruded and the solvent is rapidly evaporated causing individual filaments to assume a highly fibrillar form before they are deposited on a screen. The web is thermal bonded with a hot calendar roll to form a strong fabric. About two-thirds of North American spunbonded fabrics produced are made with polypropylene. Cover stock is the largest end-use for spunbonded polypropylene and represents about two thirds of that type of spunbonded material. Other important applications for spunbonded polypropylene are medical disposables, bedding fabrics, carpet backing, geotextiles, agricultural fabrics and industrial apparel. Roofing accounts for a third of the usage of spunbonded polyester in North America. The second largest market is fabric softener substrates followed by furniture and bedding fabrics and carpet backing. Other significant end-uses are filtration media, apparel interlining, and agricultural fabrics.

WET LAID - MELT BLOWN
The melt blown process extrudes a thermoplastic, fiber forming polymer through a linear die containing 20-40 small orifices per inch of die width. Convergent streams of hot air rapidly attenuate the extruded polymer streams to form extremely fine diameter fibers. The attenuated fibers subsequently get blown by high velocity air onto a collector screen, forming a melt blown web. The fibers in the melt blown web are laid together by a combination of entanglement and cohesive sticking. Because the fibers are drawn to their final diameters while still in the semi-molten state, there is no downstream method of drawing the fibers before they are deposited onto the collector, and hence the webs exhibit low to moderate 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. Spunbonded materials are used extensively with melt blown webs to produce strong spunbonded/melt blown composite material with barrier properties. A variation of the melt blowing process combines the molten polymer stream with a stream of absorbent materials such as wood pulp fibers and super absorbent powder to give a strong, soft and absorbent web that retains the absorbent fibers and particles. The largest end-uses for monolithic melt blown materials are oilsorbents, air and liquid filtration media. Melt blown with pulp has captured a significant share of the premoistened baby wipes business in North America. During the mid-1990's cover stock developed as a major end market for spunbonded/melt blown composite.

WEB BONDING TECHNIQUES