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Artificial Fibers

Artificial Fibers

Investigation of a crime or accident scene involves the collection of evidence . This collection must be scrupulous; every piece of evidence is important in deciphering the course of events, identifying the victim, and in implicating a suspect.

One piece of evidence that can be important is fibers ; the material that makes up clothing and other material. Even wigs may be comprised of fibers.

Fibers can be made of a natural material (i.e., wool) or a synthetic compound or blend. The differentiation of these fibers types can be important. For example, if a suspect was wearing rayon trousers, then it would be of interest to determine if the fibers found at the scene were rayon.

Most synthetic fibers are polymer-based, and are produced by a process known as spinning. This process involves extrusion of a polymeric liquid through fine holes known as spinnerets. After the liquid has been spun, the resulting fibers are oriented by stretching or drawing out of the fibers. This increases the polymeric chain orientation and degree of crystallinity, and has the effect of increasing the modulus and tensile strength of the fibers.

Fiber manufacture is classified according to the type of spinning that the polymer liquid undergoes: melt spinning, dry spinning, or wet spinning.

Melt spinning is the simplest of these three methods, but it requires that the polymer constituent be stable above its melting temperature. In melt spinning, the polymer is melted and forced through the spinnerets, which may contain from 50 to 500 holes. The diameter of the fiber immediately following extrusion exceeds the hole diameter. During the cooling process, the fiber is drawn to induce orientation. Further orientation may later be achieved by stretching the fiber to what is known as a higher draw ratio.

Melt spinning is used with polymers such as nylon, polyethylene, polyvinyl chloride, cellulose triacetate, and polyethylene terephthalate, and in the multifilament extrusion of polypropylene.

In dry spinning, the polymer is first dissolved in a solvent. The polymer solution is extruded through the spinnerets. The solvent is evaporated with hot air and collected for reuse. The fiber then passes over rollers, and is stretched to orient the molecules and increase the fiber strength. Cellulose acetate, cellulose triacetate, acrylic, modacrylic, aromatic nylon, and polyvinyl chloride are made by dry spinning.

In wet spinning, the polymer solution is spun into a coagulating solution to precipitate the polymer. This process has been used with acrylic, modacrylic, aromatic nylon, and polyvinyl chloride fibers. Viscose rayon is produced from regenerated cellulose by a wet spinning technique.

In a forensic examination, fibers are most easily collected using adhesive tape. The collected fibers are separated based on color and other appearance characteristics (i.e., wooly versus string-like).

Forensic analysis of fibers is conducted in several ways. Synthetic fiber polymers can be suited to examination using infrared spectroscopy . Specified guidelines exist for this type of examination, which makes the technique standard and so more easily legally admissible.

The constituents of the dye that has been used to color fibers can be separated using chromatography , which can separate compounds based on differences of size or charge.

Artificial fibers can also act as lenses, by virtue of the drawing out process of manufacture. Based on the optical properties of a fiber, shining a light on it will either focus the light towards the center or the edge of the fiber. This can aid in identifying the nature of a fiber sample.

see also Crime scene investigation; Evidence; Fourier transform infrared spectrophotometer (FTIR).

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Artificial Fibers

Artificial fibers

An artificial fiber is a threadlike material invented by human researchers. Such fibers do not exist naturally. Some examples of artificial fibers include nylon, rayon, Dacron, and Orlon. These terms illustrate that some names of artificial fibers are, or have become, common chemical names (nylon and rayon), while others (Dacron and Orlon) are proprietary names. Proprietary names are names that are owned by some company and are properly written to indicate that the name is a registered trademark ().

Most artificial fibers are polymers. A polymer is a chemical substance that is produced when one or two small molecules are reacted with each other over and over again. The beginning molecule used in making a polymer is called a monomer. When two different monomers are used, the product that results is called a copolymer.

An example of a copolymer is nylon, first invented by American chemist Wallace Carothers (18961937) in 1928. The two monomers of which nylon is made are complicated substances called adipic acid and hexamethylenediamine. For simplicity, call the first monomer A and the second monomer B. A molecule of nylon, then, has a structure something like this:

-A-B-A-B-A-B-A-B-A-B-A-B

The dashes at the beginning and end of the molecule indicate that the -A-B- sequences goes on and on until it contains hundreds or thousands of monomer units.

The usual process by which artificial fibers are produced is called spinning. When a polymer is first produced, it is generally a thick, viscous (sticky) liquid. That liquid is forced through a disk containing fine holes known as a spinneret. The spinneret may be suspended in the air or it may be submerged under water. As the polymeric liquid passes through the spinneret holes, it become solid, forming long, thin threads.

The properties of an artificial fiber can be changed in a number of ways, including the way in which the polymer is first produced, additives that may be attached to the polymer, and the way the polymer is processed through the spinneret.

Other synthetic fibers

Artificial fibers can be made by processes other than polymerization. Glass fibers, for example, can be produced by melting certain kinds of glass and then forcing the melted material through a spinneret to form long, thin threads. Many of these artificial fibers not made from polymers are the result of recent chemical research and show exciting promise for new applications in industry.

[See also Polymer ]

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