Fiber-Specific Processes

Mercerizing can be applied between various production steps  

owing to its hydrophilic behavior, is treated very differently from cotton and man-made fibers. 

Raw wool includes wool grease (lanolin), animal excrement, salt from perspiration, and rough impurities such as sand and parts of plants. The pollutants and impurities of the raw wool vary as follows depending on its origin  

The share of wool hair in the raw wool varies between 15% and 72%. 

These impurities are washed out of the loose raw wool before the spinning process (a raw wool opener is connected in series at which rough pollutants are also removed) in slightly alkaline liquor with temperatures under 38 °C. Here mechanical loads have to be avoided. The wool washing takes place mostly in sieve drum washing machines that consist of four to seven flat vats in which the raw wool swims at the surface of the washing liquor (Fig. 9.5). The first vat is the so-called soaking or anti-sweat vat and the last one is the dish vat. 

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More than 95% of current carbon fiber production for advanced composite appHcations is based on the thermal conversion of polyacrylonitrile (PAN) or pitch precursors to carbon or graphite fibers. Generally, the conversion of PAN or pitch precursor to carbon fiber involves similar process steps fiber formation, ie, spinning, stabilization to thermoset the fiber, carbonization—graphitization, surface treatment, and sizing. Schematic process flow diagrams are shown in Eigure 4. However, specific process details differ. 

In most materials selection processes, it is virtually impossible to make materials choices independent of the product shape. This includes not only the macroscopic, or bulk, shape of the object such as hammer or pressure relief valve, but also the internal or microscopic shape, such as a honeycomb structure or a continuous-fiber-reinforced composite. Shape is so important because in order to achieve it, the material must be subjected to a specific processing step. In Chapter 7, we saw how even simple objects made from a single-phase metal alloy could be formed by multiple processes such as casting or forging, and how these processing steps can affect the ultimate properties of the material. As illustrated in Figure 8.6, function dictates the choice of... 

Transfer Dyes. In transfer printing the dye is supplied in the form of a coating on transfer paper. The fabric is pressed closely against the paper, and the dye is sublimed at ca. 200°C and diffuses into fibers. This process is used primarily for printing on polyester fabrics. Originally it relied on available disperse dyes with good sublimation characteristics. New dyes, specifically developed for this process, have appeared on the market recently (see also [16, vol. VIII, pp. 191 ff.]). 

A porous system resulting from dispersion of a (macroscopically) continuous medium, condensation, a chemical reaction, or from any other specific process (e.g. physical or biological) may be called a growth system. Such a system usually possesses an inimitable morphology. Growth systems include the following natural or man-made porous materials pumice, cokes, activated carbon, carbon, ceolites, cellulose fibers, and finally most foamed polymers. 

Although the actual composition depends on the specific process as well as the soybean variety, fresh okara contains 76—80% moisture, 2.6-4.0% protein, and the remaining percentages for other solids. When dried, okara contains 25.4-28.4% protein, 9.3—10.9% oil, 40.2-43.6% insoluble fiber, 12.6-14.6% soluble fiber, and... 

Special types of glass fiber for specific processes (Chapters 4 and 5), such as sheet molding compound (SMC), pultrusion, and reinforcement of thermoplastics, is a marked trend in current development. Examples follow ... 

There are several process requirements for the preparation of polypropylene staple with permanent three-dimensional helical curvature. Specifically, a rectangular spinneret-pack assembly is used to produce flow perturbation and to impart high internal stress. A specially designed cooling device cools the fiber quickly to form a paracrystalline structure in the fiber. The process principle is that the flow perturbed in the polypropylene melt creates internal stress on one side of the fiber section. Because of the stress memory of polypropylene, the internal stress difference at the interface of streamlined and perturbed flows can sustain in the fiber after it has been cooled and solidified. This leads to different crystal structures and shrink properties, and thus a fiber in the shape of a three-dimensional helix. 

Unsupported carbon fibers are formed in a continuous process [47-48] where the metal catalyst particles are continuously mixed into the flow of the feed gases, and where fibers carrying a metal catalyst particle in their tips are continuously removed. A specific process [51] that is currently under commercial development uses iron pentacarbonyl as a catalyst and hydrocarbons such as methane, natural gas, or others which can be derived from coal, recyclate and discarded rubber tires. These fibers are only several pim long and have much lower diameters (0.1 to 0.2 jm). Apparently, less time is available for side growth (or thickening) in a continuous process. 

Carbon fiber production processes are proprietary. In some instances, the plant is designed and constructed in-house, alternatively it is purchased from a specific manufacturer with, or without, input from the purchaser. Such input automatically confers confidentiality, so it is not possible to give exact design details, but a compromise has been taken here and guidelines are given for a PAN production process, the pros and cons of alternative techniques being discussed below. 

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