Processing of Thermoset Resins

J. M. Methven and I Katramados (32) used die dielectric measurements and friction methods for monitoring a pultusion process of thermoset resin in line. The dielectric measurement was used in order to determine the resin s crosslinking. 

This technique is well-suited to the manufacture of cylindrical containers of low thickness. A variant of this technique, called molding by compression transfer, is widely used for the processing of thermosetting resins. It involves the preheating of the material to be molded in a simple mold ( pot of transfer ), permitting its fast thermal transfer then the matter of appropriate fluidity is introduced under pressure into the compression mold, and the curing of the thermosetting resin occurs. In this case, the amount of matter transferred into the mold should not be in excess. 

Extrusion is one of the most used processing methods. It is well-suited to the production of tubes, pipes, sheets, and filaments. The polyma- melt is extruded with extremely high pressure through the nozzle of a die whose cross-sectional shape determines those of the resulting parison. Extrusion applies preferentially to thermoplastic materials, but it can also be adapted to the processing of thermosetting resins. 

A variety of thermosetting resins are used in SMC. Polyesters represent the most volume and are available in systems that provide low shrinkage and low surface profile by means of special additives. Class A automotive surface requirements have resulted in the development of sophisticated systems that commercially produce auto body panels that can be taken direcdy from the mold and processed through standard automotive painting systems, without additional surface finishing. Vinyl ester and epoxy resins (qv) are also used in SMC for more stmcturaHy demanding appHcations. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Kardos,J. L., Dudukovic, M. P., Dave,R. Void Growth and Resin Transport During Processing of Thermosetting — Matrix Composits. Vol. 80, pp. 101 — 123. 

Because most thermoset composites cure by a thermally activated reaction, a complicated heat transfer process occurs during solidification, the result of an exothermic cross linking reaction in the resin. The complications of thermoset resin curing are compounded by the competing mechanisms of... 

The polymerization of cyclic low-molar-mass polycarbonates, polyarylates, and PBT to high-molar-mass thermoplastics has been extensively studied by the General Electric Company during the last decade.57,58 Due to very low viscosity, cyclic oligoesters can be processed like thermosetting resins but retain thermoplastic properties in the final state, after polymerization in the presence of suitable... 

Accelerate chemical, photochemical, biochemical reactions or processes, e.g. cross-linking or degradation of polymers. Also called promoters, co-catalysts. Refer usually to the cure process in thermosetting resins. 

Transfermium elements, names of, 17 387t Transfer molding, 10 458 of Teflon PFA, 18 337 of thermosetting resins, 19 557-558 Transfer of copyrights, 7 788-791 Transfer panels, 11 33-34 Transfer printing, 9 221, 242 Transfer processes, relationship among, 15 731... 

A substantial effort in our laboratory has been directed toward the synthesis and characterization of acetylene-terminated (AT) matrix resins. The most significant feature and driving force for the effort is that the thermal induced addition reaction provides a moisture Insensitive cured product. This technology offers a wide variety of thermoset resins for various high temperature applications. Backbone structural design for use temperature capabilities, processing characteristics and mechanical performance has demonstrated the versatility of the AT type systems. 

The mechanism for cross-linking of thermosetting resins is very complex because of the relative interaction between the chemical kinetics and the changing of the physical properties [49], and it is still not perfectly understood. The literature is ubiquitous with respect to studies of cure kinetic models for these resins. Two distinct approaches are used phenomenological (macroscopic level) [2,5,50-72] and mechanistic (microscopic level) [3,73-85]. The former is related to an overall reaction (only one reaction representing the whole process), the latter to a kinetic mechanism for each elementary reaction occurring during the process. 

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

Void Growth and Resin Transport During Processing of Thermosetting — Matrix Composites... 

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