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47 GLOSSARY OF TERMS AcceleratorâSee Initiator. CatalystâSee Initiator. Cross-linking agentâBifunctional or polyfunctional monomer whose addition to a polymer system increases the rigidity, resistance to solvents, and softening point of the polymer. Curing agentâSee Initiator and Hardener. Epoxy resinâA resin that contains or did contain epoxy groups principally responsible for its polymerization. Flash pointâThe lowest temperature at which the vapor of a combustible liquid can be made to ignite momentarily in air. Glass transition temperature (Tg)âThe temperature at which an amorphous material (such as glass or a high polymer) changes from a brittle, vitreous state to a plastic state. HardenerâThe chemical component added to epoxy resins that causes the resin to harden or cure. InhibitorâFree-radical scavengers added to monomers to react with and deactivate the free radicals in growing polymer chains, and to act as antioxidants to prevent polymerization by oxidation product reaction during monomer storage. InitiatorâAgent that initiates growth of polymer chains by decomposing into free radicals that actually start the chainâs growth; often incorrectly called a catalyst. MicrocracksâSmall, numerous, noncritical cracks that can develop in hardened concrete in the matrix and at matrixâ aggregate interfaces both prior to the concrete receiving external loads or during loading. MonomerâA small molecule from which much larger poly- mer molecules can be made; usually in liquid form for concrete applications. Monomer depletionâThe loss of monomer because of evaporation from the surface of hardened concrete prior to the polymerization process associated with the produc- tion of polymer-impregnated concrete. Multiple-layer overlayâTwo or more layers of polymer concrete bonded to concrete; normally each layer con- sists of an application of resin with aggregate broadcast into the surface. PlasticizerâChemical additions to monomers to improve the flexibility of inherently brittle polymers. PolymerâThe product of polymerization, more commonly a rubber or resin consisting of large molecules formed by polymerization. PolymerizationâThe reaction in which two or more mole- cules of the same substance combine to form a compound containing the same elements, and in the same propor- tions, but of high molecular weight, from which the origi- nal substance can be regenerated, in some cases only with extreme difficulty. Polymer concrete (PC)âA composite material in which the aggregate is bound in a matrix with a polymer binder. PorosityâThe ratio, usually expressed as a percentage, of the volume of voids in a material to the total volume of the material, including the voids. Premix placementâThe method of initially blending a polymer binder, with fine and coarse aggregate and fill- ers, if used, and then mixing until all particles are com- pletely wetted. Once the composite has been mixed as required, it is transported and placed. Term applies to polymer concrete. Prepackaged polymer concreteâPolymer concretes whose individual components (that is, monomer or resin, fillers, and aggregates) are premeasured and packaged by the manufacturer for a prescribed sequence of introduction into the mixing process. Promoted-catalytic methodâA polymerization method that uses promoters or accelerators to cause the decompo- sition of organic peroxide initiators, and subsequent release of free radicals that allow polymerization to take place at ambient temperature without the need for an external source of energy. PromoterâSee Initiator. ResinâA natural or synthetic, solid, or semisolid organic material of indefinite and often high molecular weight, with a tendency to flow under stress. It usually has a softening or melting range and usually fractures conchoidally. Shelf lifeâMaximum interval during which a material may be stored and remain in a usable and safe condition. Silane coupling agentâSilicon compounds having the gen- eral formula (HO)3SiR were R is an organic group compat- ible with thermoplastic or thermosetting resins. They are used to enhance the chemical bond of organic polymers to inorganic materials such as sand, rock, glass, and metals. Slurry overlayâOverlay applied by placing an application of resin or monomer followed by broadcasting aggregate onto the surface.
48 SolventâA liquid capable of dissolving another substance. Thermal-catalytic methodâA polymerization method that uses chemical initiators that are dissolved in the mono- mer before introducing the mixture into the concrete. The subsequent application of external heat then causes the polymerization to occur at a rapid rate. ThermosettingâTerm applied to synthetic resins that solid- ify or set on heating or curing and cannot be remelted. Thin polymer overlays (TPOs)âOne or more layers of polymer concrete bonded to concrete, normally 1 in. or less in thickness. ViscosityâFriction within a liquid owing to mutual adher- ence of its particles. Low-viscosity liquid monomers flow more easily into the pores of concrete at ambient tempera- tures and pressures than do high-viscosity monomers.