Fibers continuous

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

A composite material (1) is a material consisting of two or more physically and/or chemically distinct, suitably arranged or distributed phases, generally having characteristics different from those of any components in isolation. Usually one component acts as a matrix in which the reinforcing phase is distributed. When the continuous phase or matrix is a metal, the composite is a metal-matrix composite (MMC). The reinforcement can be in the form of particles, whiskers, short fibers, or continuous fibers (see Composite materials). 

There are three kinds of metal-matrix composites distinguished by type of reinforcement particle-reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or sheet-reinforced MMCs. Table 1 provides examples of some important reinforcements used in metal-matrix composites as well as their aspect (length/diameter) ratios and diameters. 

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

Fig. 10. A dark field (DF) transmission electron micrograph showing interface in a continuous fiber (F) a-Al202 (F)/Mg alloy (ZE41A) matrix (M) within...

In aerospace appHcations, low density coupled with other desirable features, such as tailored thermal expansion and conductivity, high stiffness and strength, etc, ate the main drivers. Performance rather than cost is an important item. Inasmuch as continuous fiber-reinforced MMCs deUver superior performance to particle-reinforced composites, the former are ftequendy used in aerospace appHcations. In nonaerospace appHcations, cost and performance are important, ie, an optimum combination of these items is requited. It is thus understandable that particle-reinforced MMCs are increa singly finding appHcations in nonaerospace appHcations. 

Composites. High molecular weight PPS can be combiaed with long (0.6 cm to continuous) fiber to produce advanced composite materials (131). Such materials having PPS as the polymer matrix have been developed by usiag a variety of reinforcements, including glass, carbon, and Kevlar fibers as mat, fabric, and unidirectional reinforcements. Thermoplastic composites based on PPS have found application ia the aircraft, aerospace, automotive, appliance, and recreation markets (see Composite materials, polymer-matrix). 

There are many different equipment options avaQable to suit specific product needs including continuous winders for pipe, multiaxis winders for pressure vessels, and simple lathe-type winders for tanks and large pipe. Specialty machines combine a chopped reinforcement with continuous fibers for tank walls and large-diameter pipe where both stiffness and tensQe strength are required. Textile braiders have also been adapted for use as continuous... 

Fig. 9. Schematic representation of pultmsion process for making continuous fiber rods.

Fig. 14. Failure mechanisms for continuous fiber reinforced ceramic matrices (58).

Ceramic matrix composites are candidate materials for high temperature stmctural appHcations. Ceramic matrices with properties of high strength, hardness, and thermal and chemical stabiUty coupled with low density are reinforced with ceramic second phases that impart the high toughness and damage tolerance which is required of such stmctural materials. The varieties of reinforcements include particles, platelets, whiskers and continuous fibers. Placement of reinforcements within the matrix determines the isotropy of the composite properties. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

The preceding analysis is premised on having continuous fibers of equal strength all of which fracture at the same longitudinal position. However, fibers under tension do not all have the same fracture strength nor do they fracture in the same place. Rather, because surface imperfections vary from fiber to fiber, the individual fibers have different fracture strengths. A statistical analysis is then necessary to rationally define the strength of a composite material. 

The conclusion that short stable fibers will not produce maximum physical properties is not theoretically correct. Both experiment and theory have concluded that with proper adhesion or bond between fibers and plastic matrix, maximum properties can basically be achieved by using relatively short stable fibers rather than continuous filament construction (39). To date the higher performances is overwhelming achieved with the continuous fibers. Also, the fibers used in RPs have the important potential of reaching values that are far superior (7,10). 

Fiber denier It is a unit of weight expressing the size or coarseness but particularly the fineness of a continuous fiber or yarn. The weight in grams of 9000 m (30,000 ft) is one denier. The lower the denier, the finer the fiber, yam, etc. One denier equals about 40 micron. Sheer women s hosiery usually runs 10 to 15 denier. Commercial work 12 to 15 denier fiber is generated. 

A composite material is defined as a material consisting of two or more distinct constituents or phases, which are insoluble in one another. The main types of reinforcement are particles, discontinuous fibers, continuous fibers (or filaments) and flakes. 

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