Air-laid webs

Figure 8.14 Air-laid webs with local surface projections (Enloe, 1988). 

Dry-Laid Pulp. A principal objective of using air to form webs from natural and synthetic fiber pulps is to produce relatively lofty, porous stmctures from short fibers, without using water. Early technical developments in air-laid pulp processing were made by Kroyer in Denmark. 

Air-laid pulp-forming lines generally consist of three or more forming heads ia tandem. Liae widths range from 1 to 3 m and operate at speeds of some 400 m /min. Web weights range from 70 to 2000 g/m at throughputs of about 1000 kg/h. Air-laid pulp lines can be modified to process mixtures of textile and pulp fibers and to accommodate the addition of particulate matter. 

Air-laid nonwovens n. Fabrics made by an air-forming process. The fibers are distributed by air currents to give a random orientation within the web and a fabric with isotropic properties. 

Dry-laid n. Non-woven web made from dry fiber. Usually refers to fabrics from carded webs versus air-laid non-wovens, which are formed from random webs. 

Nonwoven Manufacturing. Cotton staple is readily processed to form carded, air laid, or carded/crossed-lapped webs that can be bonded by various techniques to form useful nonwoven materials, eg, needlepunched, spimlaced (hydroentangled), and stitch-bonded nonwovens, and resin-bonded and thermal-bonded carded fabrics (68). Many times a combination of these processes is used to produce hybrid structures and other products. Cotton s share of the nonwovens market in 2002 is 7.8% globally and 2.8% in North America (69). In 2000-2001, 32-36 million kg (70-80 million lb) of cotton was used in North America to... 

Air-laid pulp-forming lines generally consist of three or more forming heads in tandem. Web weights range 70-2000 g/m at throughputs of about 1000 kg/h. 

Hydrogen and friction bonded cellulose wherein carded, air laid or wet laid fiber web is hydroentangled. By controlling the process conditions, it should be possible to develop a structure that is dispersible in water imder the suggested conditions. 

Fibre alignments in nonwoven fabrics are usually anisotropic, ie, the number of fibres in each direction in nonwoven fabrics is not equal. The most significant difference of these properties is the differences between the fibre orientation in the fabric plane and in the direction perpendicular to the fabric plane (ie, transverse direction or fabric thickness direction) in most nonwoven webs except air-laid nonwovens, most of the fibres are preferentially aligned in fabric plane rather than in fabric thickness. There are also significant differences of fibre orientation, which are also found in the fabric MD and in the fabric CD in the nonwoven fabric plane. 

Nonwoven filter media are usually produced by using one of six major types of nonwoven web formation methods air-laid, wet-laid, spunbonding, melt blowing,... 

In the spunbond process (Fig. 10), an aspiratory is used to draw the fibers in spinning and directiy deposit them as a web of continuous, randomly oriented filaments onto a moving conveyor belt. In the meltblown process (Fig. 11), high velocity air is used to draw the extmded melt into fine-denier fibers that are laid down in a continuous web on a collector dmm. 

A recent development of the dry-laid techniques can be ascribed to the preformed GMT produced by a fluidized bed process [3]. In this case, fiber rovings are cut and mixed with thermoplastic powder, conveyed and deposited by air stream onto a mold. The loose web formed on this mold is heated by hot gas stream prior to being transferred to a matched die compression mold. So, in this case, the web (i.e. the GMT preform) is consolidated and formed simultaneously in the compression molding... 

In the dry laid process, the conventional staple fibers are used, which are usually 12 to 200 mm or longer. The fibrous web is prepared using the classical textile carding machine or air laying machine to separate and orient... 

In the extrusion die coater, the coating layer is formed and calibrated in the slot and laid down on the substrate. The thickness is independent of the web thickness and can be reduced by stretching. There is no penetration through open substrates like fribrics, and there is no influence of particles on the coating. Good contact to the substrates can be provided by an air knife or an electrostatic field. 

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