Dry laid process

Dry-Laid Processes. These include mechanical, eg carded, and aerodynamic, eg air-laid routes. Dry-laid nonwovens are made with staple fiber processing machinery such as cards and gametts, which are designed to manipulate staple fibers in the dry state. Also included in this category are nonwovens made from filaments in the form of tow, and fabrics composed of staple fibers and stitching filaments or yams, ie, stitchbonded nonwovens. 

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... 

Four basic methods are used to form a web, and non-wovens are usually referred to by one of these methods dry-laid, spun-laid, wet-laid and other techniques. Carding, garnetting and air-laying are examples of the dry-laid processes. The dry-laid processes provide maximum product versatility, since most textile fibers and bonding systems can be utilized and conventional textile fiber processing equipment can be readily adapted with minimum additional investment. 

Within the plane of a nonwoven material, the fibers may be either completely isotropic or there may be a preferred fiber orientation or alignment usually with respect to a machine or processing direction. In the case of thicker dry-laid nonwovens, fiber orientation may be randomized in the third dimension, ie, that dimension which is perpendicular to the plane of the fabric, by a process known as needle-punching (7). This process serves to bind the fibers in the nonwoven by mechanical interlocking. 

Nonwoven Fabric. Crimped PVA staple is being used for the manufacture of dry-laid nonwoven. Also, as an example utilising the uniqueness of the fiber, a soft sheet is prepared by shrinking and pardy dissolving in hot water a nonwoven from water-soluble PVA fiber and then insoliibili ing the fabric by acetalization or similar processes. This sheet is used as car wipers, wipers for high grade furniture, and for similar purposes. 

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. 

The macroporous fibrous matrix is either dry laid, meltblown, or wet laid. The wet laid process is very similar to the papermaking process. The fibers are bonded together by chemical or thermal bonding. The meltblown process is a binderless process and there the polymer fiber web is extruded. Typical pore sizes of the fibrous matrix vary from 1 to 100 fim. 

Techniques commonly used to produce fibrous webs include the wet laid, dry laid carded, and meltblown processes. The wet laid or paper making process is the predominant method for several reasons. The wet laid process, configured properly, allows for the blending of cellulosic and polymeric components. Also, the ability to use short cut length and fine denier fiber provides for consistent blending, uniform formation, and controlled pore structure. 

Non-woven The AQUATHANES inherently have excellent water and oil resistance in technical wet and dry laid application. They show further process aid in their release properties from surface contact in web formation and drying. 

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... 

Dry laid media manufacturing methods are based on traditional opening and carding processes used in the textile industry the media are composed of short fibres. Multiple layers of the open fibres are laid mechanically to form... 

The basic concept employed in making a nonwoven fabric is to transform fiber-based materials into two-dimensional sheet structures with fabric-like properties. These are flexibility, porosity, and mechanical integrity. Their manufacturing processes can be split into four groups dry-laid webs, extrusion-formed webs, wet-laid webs, and web-bonding. 

Staple fibre grades are produced to suit carded dry laid, air laid and wet laid processes. The attributes of lyocell fibre are discussed with reference to each of these conversion technologies below. 

DWI s manufacturing process relies on standard wet-laid processing. The materials are blended uniformly, and then fed into a headbox at very high dilution. The water is removed, and the web is dried. The materials are polymers selected for the application. In Titanium, the combination is chosen for alkaline resistance as well as high porosity. In Silver, polyacrylonitrile nanohbers are blended with cellulose to achieve maximum cycle life, rate capability, and minimum pore size, which also achieve advanced flame-resistant properties. In Gold, the backbone is para-aramid Twaron, which gives superior thermal stability. 

Dry-laid needle-punch technology is based on a two-step process. Firstly, PP resin is extruded into fibres. Secondly, the fibres are carded and needle-punched. Weights range from 100 g/m up to 1200 g/m. Dry-laid needle punch technology is claimed to ensure high quality geotextiles with ... 

The use of nearly all types of fiber webs is possible, but mainly dry-laid webs are used for this process. A carrying sieve transports the web through the installation so that it arrives in the bonding region, where columnar water impacts on the web with high pressure out of thin jets at a short distance. The fibers are swirled with each other. After the penetration of the web and the base, the water of this process is sucked off through vacuum seals. Figure 6.24 depicts the principle of this construction. 

Detailed description of these technologies can be found elsewhere [1,4]. As seen in Fig. 10-6, spun-laid and dry-laid are two preferred processes both in North America and Europe. The spun-laid process showed the strongest growth during a last decade [1, 7]. Products produced by the spun-laid technique account for 20-25 % of the market in Japan and China [5, 6]. 

Fig. 10-6 Comparison of web formation technologies in North America and Europe. Spun-laid and dry-laid are two preferred processes both in North America and Europe.

The differences between the two main classes of dry-laid spun material are sig-nihcant in terms of hltration behaviour, but both are available with the same range of hnishing processes as are used for woven and needlefelt materials calendering, singeing and coating. The lamination of different materials is also an important fea-tnre of dry-laid spnn media. 

High-shock grades cannot be proeessed on mills or other intensive mixers without destroying the essential fibrous structure of the filler. In these cases a wet process is used in which the resin is dissolved in a suitable solvent, such as industrial methylated spirits, and blended with the filler and other ingredients in a dough mixer. The resulting wet mix is then laid out on trays and dried in an oven. 

All these plants are of the chamber type (see Fig. 1.88 (c)) with cooled and heated shelves and a condenser which can be separated from the drying chamber by a valve. Refrigeration and vacuum systems should be laid out for temperatures and pressures which can be expected under extreme experimental conditions, even if these extreme data may not be used in the production process. Pilot plants for pharmaceutical or medical products should be laid out differently than those used for food. 

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