(See related pages)
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| Thermoplastic (noun) | a plastic material that requires heat to make it formable (plastic)
and upon cooling, retains its shape. Thermoplastics are composed of chain
polymers with the bonds between the chains being of the secondary permanent
dipole type. Thermoplastics can be repeatedly softened when heated and harden
when cooled. Typical thermoplastics are polyethylenes, vinyls, acrylics, cellulosics,
and nylons. (See page(s) 365, Sec. 7.1) |
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| Thermosetting plastic (thermoset) | a plastic material that has undergone a chemical
reaction by the action of heat, catalysis, etc., leading to a cross-linked network
macromolecular structure. Thermoset plastics cannot be remelted and reprocessed
since when they are heated they degrade and decompose. Typical thermoset plastics
are phenolics, unsaturated polyesters, and epoxies. (See page(s) 366, Sec. 7.1) |
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| Monomer | a simple molecular compound that can be covalently bonded together to
form long molecular chains (polymers). Example: ethylene. (See page(s) 366, Sec. 7.2) |
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| Chain polymer | a high-molecular-mass compound whose structure consists of a large
number of small repeating units called mers. Carbon atoms make up most of the main-chain
atoms in most polymers. (See page(s) 366, Sec. 7.2) |
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| Mer | a repeating unit in a chain polymer molecule. (See page(s) 366, Sec. 7.2) |
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| Polymerization | the chemical reaction in which high-molecular-mass molecules are
formed from monomers. (See page(s) 366, Sec. 7.2) |
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| Copolymerization | the chemical reaction in which high-molecular-mass molecules are
formed from two or more monomers. (See page(s) 366, Sec. 7.2) |
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| Chain polymerization | the polymerization mechanism whereby each polymer molecule
increases in size at a rapid rate once growth has started. This type of reaction occurs in
three steps: (1) chain initiation, (2) chain propagation, and (3) chain termination. The
name implies a chain reaction and is usually initiated by some external source. Example:
the chain polymerization of ethylene into polyethylene. (See page(s) 366, Sec. 7.2) |
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| Degree of polymerization | the molecular mass of a polymer chain divided by the molecular
mass of its mer. (See page(s) 366, Sec. 7.2) |
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| Functionality | the number of active bonding sites in a monomer. If the monomer has
two bonding sites, it is said to be bifunctional. (See page(s) 366, Sec. 7.2) |
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| Homopolymer | a polymer consisting of only one type of monomeric unit. (See page(s) 366, Sec. 7.2) |
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| Copolymer | a polymer chain consisting of two or more types of monomeric units. (See page(s) 366, Sec. 7.2) |
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| Cross-linking | the formation of primary valence bonds between polymer chain molecules.
When extensive cross-linking occurs as in the case of thermosetting resins, cross-linking
makes one supermolecule of all the atoms. (See page(s) 366, Sec. 7.2) |
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| Stepwise polymerization | the polymerization mechanism whereby the growth of the
polymer molecule proceeds by a stepwise intermolecular reaction. Only one type of
reaction is involved. Monomer units can react with each other or with any size polymer
molecule. The active group on the end of a monomer is assumed to have the same
reactivity no matter what the polymer length is. Often a by-product such as water is
condensed off in the polymerization process. Example: the polymerization of nylon
6,6 from adipic acid and hexamethylene diamine. (See page(s) 366, Sec. 7.2) |
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| Bulk polymerization | the direct polymerization of liquid monomer to polymer in a reaction
system in which the polymer remains soluble in its own monomer. (See page(s) 366, Sec. 7.3) |
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| Solution polymerization | in this process a solvent is used that dissolves the monomer,
the polymer, and the polymerization initiator. Diluting the monomer with the solvent
reduces the rate of polymerization, and the heat released by the polymerization reaction
is absorbed by the solvent. (See page(s) 366, Sec. 7.3) |
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| Suspension polymerization | in this process water is used as the reaction medium, and
the monomer is dispersed rather than being dissolved in the medium. The polymer
products are obtained in the form of small beads that are filtered, washed, and dried in
the form of molding powders. (See page(s) 366, Sec. 7.3) |
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| Crystallinity (in polymers) | the packing of molecular chains into a stereoregular
arrangement with a high degree of compactness. Crystallinity in polymeric materials
is never 100 percent and is favored in polymeric materials whose polymer chains are
symmetrical. Example: high-density polyethylene can be 95 percent crystalline. (See page(s) 367, Sec. 7.4) |
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| Glass transition temperature | the center of the temperature range where a heated
thermoplastic upon cooling changes from a rubbery, leathery state to that of brittle
glass. (See page(s) 367, Sec. 7.4) |
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| Stereoisomers | molecules that have the same chemical composition but different structural
arrangements. (See page(s) 367, Sec. 7.4) |
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| Atactic stereoisomer | Atactic stereoisomer: this isomer has pendant groups of atoms randomly arranged
along a vinyl polymer chain. Example: atactic polypropylene. (See page(s) 367, Sec. 7.4) |
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| Isotactic isomer | this isomer has pendant groups of atoms all on the same side of a vinyl
polymer chain. Example: isotactic polypropylene. (See page(s) 367, Sec. 7.4) |
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| Syndiotactic isomer | this isomer has pendant groups of atoms regularly alternating
in positions on both sides of a vinyl polymer chain. Example: syndiotactic polypropylene. (See page(s) 367, Sec. 7.4) |
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| Stereospecific catalyst | a catalyst that creates mostly a specific type of stereoisomer during
polymerization. Example: the Ziegler catalyst used to polymerize propylene to
mainly the isotactic polypropylene isomer. (See page(s) 367, Sec. 7.4) |
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| Injection molding | a molding process whereby a heat-softened plastic material is forced
by a screw-drive cylinder into a relatively cool mold cavity that gives the plastic the
desired shape. (See page(s) 367, Sec. 7.5) |
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| Blow molding | a method of fabricating plastics in which a hollow tube (parison) is
forced into the shape of a mold cavity by internal air pressure. (See page(s) 367, Sec. 7.5) |
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| Extrusion | the forcing of softened plastic material through an orifice, producing a continuous
product. Example: plastic pipe is extruded. (See page(s) 367, Sec. 7.5) |
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| Compression molding | a thermoset molding process in which a molding compound
(which is usually heated) is first placed in a molding cavity. Then the mold is closed
and heat and pressure are applied until the material is cured. (See page(s) 367, Sec. 7.5) |
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| Transfer molding | a thermoset molding process in which the molding compound is first
softened by heat in a transfer chamber and then is forced under high pressure into one
or more mold cavities for final curing. (See page(s) 367, Sec. 7.5) |
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| Plasticizers | chemical agents added to plastic compounds to improve flow and processibility
and to reduce brittleness. Example: plasticized polyvinyl chloride. (See page(s) 367, Sec. 7.6) |
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| Filler | a low-cost inert substance added to plastics to make them less costly. Fillers may
also improve some physical properties such as tensile strength, impact strength,
hardness, wear resistance, etc. (See page(s) 367, Sec. 7.6) |
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| Elastomer | a material that at room temperature stretches under a low stress to at least
twice its length and then quickly returns to almost its original length upon removal of
the stress. (See page(s) 367, Sec. 7.9) |
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| cis-1,4 polyisoprene | the isomer of 1,4 polyisoprene that has the methyl group and
hydrogen on the same side of the central double bond of its mer. Natural rubber
consists mainly of this isomer. (See page(s) 367, Sec. 7.9) |
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| trans-1-4 polyisoprene | the isomer of 1,4 polyisoprene that has the methyl group and
hydrogen on opposite sides of the central double bond of its mer. (See page(s) 367, Sec. 7.9) |
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| Vulcanization | a chemical reaction that causes cross-linking of polymer chains. Vulcanization
usually refers to the cross-linking of rubber molecular chains with sulfur, but
the word is also used for other cross-linking reactions of polymers such as those that
occur in some silicone rubbers. (See page(s) 367, Sec. 7.9) |