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GLOSSARY OF TERMS 119 GLOSSARY OF TERMS Amorphous plasticA plastic that has no crystalline component. There is no order or pattern to the distribution of the molecules. Anisotropic Not isotropic. Exhibiting different properties when tested along axes in different directions. Aramid A type of highly oriented organic material derived from a polyamide with at least 85 mole percent of its amide linkages directly connected to two aromatic rings. Kevlar® is an example of a para-oriented aramid, and is a high-strength high-modulus fiber. Aromatic Unsaturated hydrocarbon with one or more benzene ring structure in the molecule. Boron fiber A fiber produced by vapor deposition of elemental boron, usually onto a tungsten filament core, to impart strength and stiffness. Carbon fiber Fiber produced by the pyrolysis of organic precursor fibers, such as rayon, polyacrylonitrile (PAN), and pitch, in an inert environment. The term is often used interchangeably with the term graphite; however, carbon fibers and graphite fibers differ. The basic differences lie in the temperature at which the fibers are made and heat treated and in the structures of the resulting fibers. Graphite fibers have a more ''graphitic'' structure than carbon fibers. Carbonization In carbon fiber manufacturing, it is the process step in which the heteroatoms are pyrolyzed in an inert environment at high temperature to leave a very high carbon content material (about 95 percent carbon and higher). The carbonization temperature can range up to 2000°C. Temperatures above 2000°C (up to 3000°C) are often referred to as graphitization temperatures. The range of temperature employed is influenced by precursor, individual manufacturing process, and properties desired. Coagulation bath A liquid bath that serves to harden viscous polymer strands into solid fibers after extrusion through a spinnerette. Used in wet spinning processes such as rayon or acrylic fiber manufacture.
GLOSSARY OF TERMS 120 Compression The ability of a material to resist a force that tends to crush or buckle. strength Crystalline The term crystalline applies to sections of all chemical fibers, which consist of alternate orientation crystalline and amorphous (noncrystalline) regions. These regions are influenced by manufacturing conditions and can be controlled to some extent. Crystalline orientation implies the extent to which these crystalline regions can be aligned parallel to the fiber axis. Crystalline orientation is an important factor in determining the mechanical properties of carbon fibers. Crystallinity The extent to which a polymer exists in a lattice structure. Damage tolerance In the aircraft industry, it is the ability of a structure to tolerate a reasonable level of damage or defects that might be encountered during manufacture or while in service without jeopardizing aircraft safety. E-glass A family of glasses with a calcium aluminoborosilicate composition and a maximum alkali content of 2 percent. A general-purpose fiber that is used most often in reinforced plastics and that is suitable for electrical laminates because of its high resistivity. Epoxy A polymerizable thermoset polymer containing one or more epoxide groups and curable by reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans. Filament The smallest unit of a fibrous material. The basic units formed during drawing and spinning, which are gathered into strands of fiber (tows) for use in composites. Filaments usually are of extreme length and very small diameter, usually less than 25 microns (a mil). Normally filaments are not used individually. Flexural strength The maximum stress that can be borne by the surface fibers in a beam in bending. Graphite The best representation of the ideal graphitic structure is the crystal structure of single-crystal graphite, which is hexagonal in nature. In this structure, planes of honeycomb carbon hexagons are stacked very close together in an ordered parallel fashion. The typical in-plane bond length is 1.42 Ã ,
GLOSSARY OF TERMS 121 and the separation between planes is 3. 354 à . High-modulus, pitch-based, carbon fibers come very close to achieving this three-dimensional order type of structure, but it is very difficult to achieve starting with PAN-based precursors. High modulus High-modulus organic fibers (HMOFs) are a new generation of organic fibers possessing organic fibers unusually high tensile strength and modulus but generally poor compressive and transverse properties. Unlike conventional fibers that have a chain-folded structure, all HMOF fibers have an oriented chain-extended structure. One approach to producing such a polymer is to make a structurally modified, highly oriented version of a "conventional" polymer such as high-modulus polyethylene. A second approach is to synthesize polymers that have inherently ordered rigid molecular chains in an extended conformation. Examples of the latter are liquid crystalline aramids and liquid crystalline polyesters (LCPs). Hybrid A composite laminate consisting of laminae of two or more composite material systems. A combination of two or more different fibers, such as carbon and glass or carbon and aramid, into a structure. Tapes, fabrics, and other forms may be combined; usually only the fibers differ. Induction furnace In the carbon fiber process, a carbonization furnace that utilizes induction heating to eliminate the heteroatoms from the carbon fiber structure. Heat distribution is obtained by a combination of induced heat to a furnace muffle/susceptor within the induction coil and radiation from the interior surfaces of the furnace chamber in temperatures in excess of 3000°C. Sometimes an induction furnace is used in combination with a resistance furnance to achieve the desired pyrolysis temperature (i.e., the resistance furnace may be used to expose the fiber to a temperature as high as 2000°C, and if higher temperatures are required, as may be the case in graphitization, the fiber could then be passed through an induction furnace). Laminate A composite material made up of a single ply or layer or series of layers with each layer consisting of a reinforcing fiber imbedded in a matrix. Each ply or layer is oriented in a predetermined manner in order to maximize the properties of the laminate.
GLOSSARY OF TERMS 122 Liquid crystal Liquid crystal materials are usually made up of rigid, rod-like molecules. They can become ordered in either solution or melt phase, which means that the molecules aggregate under certain conditions so that the materials are anisotropic. It is very easy to achieve high orientation with these materials during either liquid or melt extrusion, so the resulting extrudate (e.g., fiber or film) has excellent tensile properties. Matrix The essentially homogeneous resin or polymer material in which the fiber system of a composite is imbedded. Both thermoplastic and thermoset resins may be used, as well as metals, ceramics, and glasses. Mesophase An intermediate phase in the formation of carbon fiber from a pitch precursor. This is a liquid crystal phase in the form of microspheres, which, upon prolonged heating above 400°C, coalesce, solidify, and form regions of extended order. Heating to above 2000°C leads to the formation of graphite-like structure. Microstructure A structure with heterogeneities than can be seen through a microscope. Morphology The overall form of a polymer structure, that is, crystallinity, branching, molecular weight, etc. Also, the study of the fine structure of a fiber or other material, such as basal plane orientation across a carbon/graphite fiber. Pitch A high molecular weight material left as a residue from the destructive distillation of coal and petroleum products. Pitches are used as base materials for the manufacture of certain high- modulus carbon fibers and as matrix precursors for carbon-carbon composites. Polyacrylonitrile Used as a base material or precursor in the manufacture of certain carbon fibers. The fiber- (PAN) forming acrylic polymers are high in molecular weight and are produced commercially either by solution polymerization or suspension polymerization. Both techniques utilize free-radical- initiated addition polymerization of acrylonitrile and small percentages of other monomers. Commercial precursor fibers are more than 90 percent acrylonitrile based.
GLOSSARY OF TERMS 123 Polyamide A thermoplastic polymer in which the structural units are linked by amide or thio-amide groupings (repeated nitrogen and hydrogen groupings). Polyamideimide A polymer containing both amide and imide (as in polyamide) groups; its properties combine the benefits and disadvantages of both. Polyester A class of resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl- (Thermosetting) type active monomer such as styrene, methyl styrene, or diallyl phthalate. The two important commercial types are (1) liquid resins that are cross-linked with styrene and used either as impregnates for glass or carbon fiber reinforcements in laminates, filament-wound structures, and other built-up constructions, or as binders for chopped-fiber reinforcements in molding compounds; and (2) liquid or solid resins cross-linked with other esters in chopped-fiber and mineral-filled molding compounds. Polyether ether A linear aromatic crystalline thermoplastic. A composite with a PEEK matrix may have a ketone (PEEK) continuous-use temperature as high as 250°C. Polyimide (PI) A polymer produced by reacting an aromatic dianhydride with an aromatic diamine. It is a highly heat-resistant resin . Similar to a polyamide, differing only in the number of hydrogen molecules contained in the groupings. May be either thermoplastic or thermosetting. Poly (phenylene) A high carbon content (94.7 percent) polymer that has the monomeric repeat unit-(C6H4-)-. Polyphenylene A high-temperature thermoplastic useful primarily as a molding compound. Known for sulfide (PPS) chemical resistance. Polysulfone A high-temperature resistant thermoplastic polymer with the sulfone linkage, with a Tg of 190° C. Polyvinyl ChlorideA vinyl-type thermoplastic resin formed by the addition reaction of vinyl chloride monomer. Rayon A synthetic fiber made up primarily of regenerated cellulose. In the process, cellulose derived from wood pulp, cotton linters, or other vegetable matter
GLOSSARY OF TERMS 124 is dissolved into a viscose spinning solution. The solution is extruded into an acid-salt coagulating bath and drawn into continuous filaments. Rayon fibers were one of the first precursor materials to be used in the manufacture of carbon fibers. They have now been almost completely replaced by PAN and pitch fibers as starting materials for carbon fiber manufacture due to low yields, high processing costs, and limited physical property formation. Resin A solid or pseudosolid organic material usually of high molecular weight that exhibits a tendency to flow when subjected to stress. Used in reinforced plastics, to bind together the reinforcing material; the matrix. Resistance furnaceIn the carbon fiber process, a carbonization furnace that utilizes resistance heating to eliminate the heteroatoms from the carbon fiber structure. In this furnace, heat distribution is obtained by a combination of direct radiation from the resistors and reradiation from the interior surfaces of the furnace chamber to temperatures in excess of 1000°C. S-glass A magnesium aluminosilicate composition that is especially designed to provide very high tensile strength glass filaments. Sizing A coating put on the fiber, usually at the time of manufacture, to protect the surface and aid the process of handling and fabrication or to control the fiber characteristics. Most standard sizes used for aerospace-grade carbon fibers are epoxy based. Spinnerette A metal disc containing numerous minute holes used in yarn extrusion. The spinning solution or molten polymer is forced through the holes to form the yarn filaments. Stabilization In carbon fiber manufacture the process used to render the carbon fiber precursor infusible prior to carbonization. Surface treatment In carbon fiber manufacturing, surface treatment is the process step whereby the surface of the carbon fiber is oxidized in order to promote wettability and adhesion with the matrix resin in the composite. Tensile strength The pulling stress required to break a given specimen.
GLOSSARY OF TERMS 125 Thermal oxidative The resistance of a fiber, resin, or composite material to degradation upon exposure to elevated stability temperature in an oxidizing atmosphere. It is often measured as percent weight loss after exposure to a specified temperature for a set period of time. It may also be measured as the percentage of retained properties after elevated-temperature exposure. Thermoplastic Polymers capable of being repeatedly softened by an increase of temperature and hardened by a polymers decrease in temperature. Applicable to those polymers whose change upon heating is substantially physical rather than chemical and that in the softened stage can be shaped by flow into articles by molding or extrusion. Thermoset Polymers that, when cured by application of heat or chemical means, change into substantially polymers infusible and insoluble materials. Toughness A property of a material for absorbing work. The actual work per unit volume or unit mass of material that is required to rupture it. Tow An untwisted bundle of continuous filaments. A tow designated as 12K has 12,000 filaments. Whiskers Materials characterized by their fibrous, single-crystal structures, which have almost no crystalline defects. Numerous materials, including metals, oxides, carbides, halides, and organic compounds, have been prepared in the form of whiskers. They are often used to reinforce resin- and metallic-matrix composites.
GLOSSARY OF TERMS 126
