Fibers blocked

Polyurethanes represent approximately 6% of global plastic consumption, and 50% of the entire PU consumption occurs in North America and Europe. Roughly 30 /o of PUs are used for furniture and mattresses, 15 /o for vehicles, and 13%i for construction. In all of these applications, PU foams are used. In fact, over 80 /o of polyurethane applications use foam materials. Other forms of PU available include solid blocks, fibers, paints, adhesives, and coatings. 

Besides non-ideal operation, non-ideal module construction influences the module performance as well. Module performance refers to the recovery, which is the ratio of a product stream to the feed stream of the target compound, for a given product purity. The objective of manufacturing process is to produce uniform, defect free hollow fibers. Unfortunately, real membrane fibers are not uniform since small changes in production conditions as well as manufacturing tolerances result in different fiber properties. Variations in fiber dimensions (diameters, length, and membrane thickness) and fiber properties (permeability) as well as manufacturing defects (blocked fibers, pinholes) have an impact on the module performance. In particular, the variation of fiber diameters, which is one of the major drawbacks in module performance, was well... 

Blocked fibers result from imperfections in adhesive area, crushing and impurities during the manufacturing process. Blocked fibers can be distinguished into fibers which are blocked at the inlet and those which are blocked at the outlet, which are illustrated in Figures 5.18 and 5.19. 

In the latter case the blocked fibers can be considered as fibers with a recovery of zero in case of a retentate side product since the whole stream flowing into the fiber will permeate through the membrane. The concentration of the fast permeating component will be lower compared to the eoncentration in an unblocked fiber. Since the permeate mixes on the shell side with the permeate of unblocked fibers, the mixed concentration of the faster permeating component is lower than that produced by unblocked fibers which results in an enhanced driving force. This effect can compensate the recovery losses to some extent. 

If the fibers are blocked at the inlet, a fraction of the retentate leaving unblocked fibers will flow back into the blocked fibers which result in a loss of product and a lower recovery. In the case of blocked inlet fibers a permeate with higher product concentration will be produced, so that the driving force for permeation of the preferred permeating component is enhanced. Furthermore, the purity of retentate stream is limited in contrast to unblocked fibers. In a module containing blocked as... 

Figure 5.18 Membrane module with blocked fibers at the retentate outlet. The blocked fibers can be assumed as fibers with a recovery of 0.

In order to obtain an optimal separation performance various operational as well as design parameters have to be considered carefully. Here the flow through the module, which is usually counter current flow, the location of the feed and active layer are the most important parameters. During module manufacturing various detrimental effects can occur, which reduce the module performance, e.g. variations in fiber dimensions and properties (diameter, length, membrane thickness, permeability) and defects (pinholes, blocked fibers). In order to ensure a proper operation of the membrane module these effects have to be avoided. 

One more application area is composite materials where one wants to investigate the 3D structure and/or reaction to external influences. Fig.3a shows a shadow image of a block of composite material. It consists of an epoxy matrix with glass fibers. The reconstructed cross-sections, shown in Fig.3b, clearly show the fiber displacement inside the matrix. The sample can be loaded in situ to investigate the reaction of matrix and fibers to external strain. Also absorption and transmission by liquids can be visualized directly in three-dimensions. This method has been applied to the study of oil absorption in plastic granules and water collection inside artificial plant grounds. 

Acrylonitrile (AN), C H N, first became an important polymeric building block in the 1940s. Although it had been discovered in 1893 (1), its unique properties were not realized until the development of nitrile mbbers during World War II (see Elastomers, synthetic, nitrile rubber) and the discovery of solvents for the homopolymer with resultant fiber appHcations (see Fibers, acrylic) for textiles and carbon fibers. As a comonomer, acrylonitrile (qv) contributes hardness, rigidity, solvent and light resistance, gas impermeabiUty, and the abiUty to orient. These properties have led to many copolymer apphcation developments since 1950. 

A further development in the coumarin series is the use of derivatives of 3-phenyl-7-aminocoumarin ((13) where R, R = Cl or substituted amines) as building blocks for a series of light-stable brighteners for various plastics and synthetic fibers, and, as the quatemi2ed compounds, for brightening polyacrylonitrile (62). 

Spinnerette Process. The basic spinning process is similar to the production of continuous filament yams and utilizes similar extmder conditions for a given polymer (17). Fibers are formed as the molten polymer exits the >100 tiny holes (ca 0.2 mm) of each spinnerette where it is quenched by chilled air. Because a key objective of the process is to produce a relatively wide (eg, 3 m) web, individual spinnerettes are placed side by side in order that sufficient fibers be generated across the width. This entire grouping of spinnerettes is often called a block or bank, and in commercial production it is common for two or more blocks to be used in tandem in order to increase the coverage and uniformity of laydown of the fibers in the web. 

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