Technology & Engineering
Thermoset
A thermoset is a type of polymer that, once cured, cannot be reshaped or melted. This is due to the cross-linking of its molecular structure, which creates a rigid and durable material. Thermosets are commonly used in applications where heat resistance, chemical resistance, and dimensional stability are important, such as in automotive parts, electrical components, and adhesives.
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6 Key excerpts on "Thermoset"
eBook - PDF- P.E., Philip A. Schweitzer(Authors)
- 2006(Publication Date)
- CRC Press(Publisher)
3 Thermoset Polymers Thermoset polymers assume a permanent shape or set once cured. Once set, they cannot be reshaped. They are formed by a large amount of cross-linking of linear prepolymers (a small amount of cross-linking will produce elastomers) or by direct formation of networks by the reaction of two monomers. The latter is the more prominent of the two methods. It is a stepwise or condensation method that has been defined as “the reaction of two monomers to produce a third plus a by-product, usually water or alcohol.” Because in some cases a by-product is not produced, this definition is no longer exactly correct. The reaction is now referred to as a “stepwise” polymerization. When the reaction results in a by-product, it is called a “condensation reaction.” Table 3.1 lists the principal Thermoset polymers. Although fewer in number than the thermoplastic polymers, Thermosets comprise approximately 14% of the total polymer market. Compared to the thermoplasts, they are more brittle, stronger, harder, and generally more temperature resistant. Table 3.2 gives the operating temperature range of the Thermoset polymers. In addition, they offer the advantages of better dimensional stability, creep resistance, chemical resistance, and good electrical properties. Their disadvantages lie in the fact that most are more difficult to process and more expensive. Phenolics represent about 43% of the Thermoset market, making them the most widely used. They are relatively inexpensive and are readily molded with good stiffness. Most contain wood or glass-flour fillers and, on occasion, glass fibers. For molded products, usually formed by compression or transfer molding, mineral or cellulose fibers are often used as low-cost general purpose fillers, and glass fiber fillers are often used for optimum strength or dimensional stability. 3.1 Corrosion of Thermosets Unreinforced, unfilled Thermoset polymers can corrode by several mechanisms.
- Edith Turi(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Flexible thermo-setting polymers, or elastomers, are treated in Chapter 6. These thermal analysis techniques are often complemented by chemical iden-tity measurements such as infrared (IR), gas chromatography (GC), liq-uid chromatography (LC), and mass spectrometry (MS); molecular size measurements such as gel permeation chromatography (GPC); and rheological measurements. One such application area is quality control (May et al., 1978; Chen, 1978; Thomas et al., 1979; Penn, 1979; Zuc- coni, 1979). Another is the study of mechanisms and kinetics of curing (epoxies: Acitelli et al., 1971; Schneider et al., 1979; phenolics: Kato- vic, 1967a,b; Kurachenkov and Igonin, 1971; King et al., 1974; Westwood, 1975). Results of these measurement techniques are dis-cussed only where they have been used to supplement thermal analysis methods; TMA rheological techniques are described. As the name suggests, Thermosetting resins become set, i.e., infu-sible and insoluble, as a consequence of the chemical cross-linking reactions accompanying cure. Most formulations require heat for cur-ing; sometimes pressure is used to enhance flow. The resulting poly-mer, if properly processed, is a highly cross-linked, three-dimensional infinite network. The majority of Thermosets are used in filled or reinforced form to reduce cost, to modify physical properties, to act as a binder for parti-cles, to reduce shrinkage during cure, or to provide or enhance flame retardance. In general, Thermosets possess good dimensional stability, thermal stability, chemical resistance, and electrical properties.
eBook - PDFSustainable Plastics
Environmental Assessments of Biobased, Biodegradable, and Recycled Plastics
- Joseph P. Greene(Author)
- 2022(Publication Date)
- Wiley(Publisher)
When the polymer is heated the liquid polymer material forms 3-D covalent bonds between polymer molecules during the crosslinking process. The crosslinked materials cannot be melted and reshaped or recycled. Three of the most common Thermoset resins are polyester, vinyl ester, and epoxy. Polyester and vinyl ester are used in automotive components but not usu- ally in aerospace components. Epoxy is best used in aerospace applications that require high strength, high stiffness, and high thermal properties. Epoxy was better described in the automotive section of this chapter and will not be repeated here. 11 Thermoset Polymers 300 11.2.1 Polyimides Polyimides can also be used in aerospace applications. Polyimide is a polymer of imide monomers (Polyimides 2020). Polyimides can be Thermoset or thermoplas- tic resins. They have high thermal properties and can be used as fuel cells, dis- plays, and in other electrical devices. They are more expensive than epoxies and polyesters. They can be used with glass fiber to make a polyimide composite. They offer excellent mechanical properties and can be used in automotive plastics, films, laminating resins, insulation coatings, and high-temperature structural adhesives. The chemical structure is provided in Figure 11.18. The most common form of polyimides is PMR-15. The polymerization of mono- mer reaction creates a 1500 formulated molecular weight. The polymer is made into prepreg and compression molded at 600 °F (316 °C) for several minutes. It can be molded with sintering from powder. Polyimides have high thermal and mechanical properties and low friction with excellent chemical resistance. It has continuous service temperature of 550–600 °F (288–316 °C), which is higher than epoxy-based composites. BMI is a common polyimide with excellent thermal properties and easier pro- cessing conditions. Composite parts can be made with RTM or resin infiltration, vacuum bagging, or other hand-layup methods.
eBook - PDFFundamentals of Modern Manufacturing
Materials, Processes, and Systems
- Mikell P. Groover(Author)
- 2016(Publication Date)
- Wiley(Publisher)
The word is derived from the Greek words poly, meaning “many,” and meros (reduced to mer), meaning “part.” Most polymers are based on carbon and are therefore organic chemicals. As engineering materials, polymers are relatively new compared to metals and ceramics (see Historical Note 8.1 at www.wiley.com/college/groover). Polymers can be classified into three types: (1) thermoplastic polymers, (2) Thermosetting polymers, and (3) elastomers. Thermoplastic polymers (TP), also called thermoplastics, are solid materials at room temperature, but they become viscous liquids when heated to temperatures of only a few hundred degrees. This characteristic allows them to be easily and economically shaped into products. They can be subjected to this heating and cooling cycle repeat- edly without significant degradation. Thermosetting polymers (TS), or Thermosets, cannot tolerate repeated heating cycles as thermoplastics can; when initially heated, they soften and flow for mold- ing, but the elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid. If reheated, Thermosetting polymers degrade and char rather than soften. Elastomers (E) are polymers that exhibit extreme elastic extensibility when subjected to relatively low mechan- ical stress. Some elastomers can be stretched by a factor of 10 and yet completely recover to their original shape. Although their properties are quite different from Thermosets, they have a similar molecular structure that is different from the thermoplastics. In popular nomenclature, thermoplas- tics and Thermosets are known as plastics and elastomers are known as rubbers. Thermoplastics are commercially the most important of the three types, constituting around 70% of the tonnage of all synthetic polymers produced. Thermosets and elastomers share the remaining 30% about evenly. Common TP polymers include polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon.
- Joseph H. Koo(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
Basics of Polymer Matrices and Composites 112 and its ability to either homopolymerize, copolymerize, or both. This transformation process represents the line of demarcation separating the Thermosets from the thermo- plastic polymers. Crystalline thermoplastic polymers are capable of a degree of crys- talline cross-linking, but there is little, if any, of the chemical cross-linking that occurs during the Thermosetting reaction. The important beneficial factor lies in the inherent enhancement of Thermoset resins in their physical, electrical, thermal, and chemical properties due to their ability to maintain and retain these enhanced properties when exposed to severe environments. The cross-linking reaction, which occurs during the fabrication of Thermosets, also provides good adhesion to other materials. As a result, epoxy and polyester resins are used for fiber-reinforced composites, and phenolics are used for bonding fibers to brake pads, and sand for metal casting. Table 3.3 shows some comparison of thermoplastic and Thermoset resin characteristics [4]. A Thermosetting matrix is defined as a composite matrix capable of curing at some temperature from ambient to several hundred degrees and cannot be reshaped by reheating. In general, Thermosets contain two or more ingredients, a resin matrix with a curing agent that causes the matrix to polymerize (cure) at room tempera- ture. Thermosets can also be thermally cured at elevated temperature with a long curing cycle without a curing agent. Some commercially available resins (matrices) are polyester and vinyl ester, polyurea, epoxy, phenolic, bismaleimide, polyimide, cyanate ester, and phenyl triazine. Polyester and vinyl esters. Polyester matrices have had the longest period of use, with wide application in many large structural applications. They will cure at room temperature with a catalyst (peroxide), which produces an exothermic reaction.
eBook - PDFAdvanced Materials for Water Handling
Composites and Thermoplastics
- D.V. Scott(Author)
- 2000(Publication Date)
- Elsevier Science(Publisher)
CHAPTER 4 Production Technology Production techniques and technologies for moulding or forming thermoplastic and composite materials are too numerous and complex to be discussed in detail in this book. We have limited the information to a brief presentation of the main technologies used within the context of the water industry. For more detailed information the author recommends the Reinforced Plastics Handbook, published by Elsevier Advanced Technology. 4.1 Thermosets/Composite Materials The different moulding procedures described below are all designed to combine a Thermosetting resin with a glass fibre reinforcement. Except for moulding compounds such as BMC and SMC, for example, the glass fibre reinforcement is always applied in successive layers, and the characteristics of the finished laminate will depend upon the relative proportions of resin and glass, the type of glass reinforcement used, and the direction in which the fibres are laid, the number of layers of each type of material, the characteristics of the resin used, and the type and duration of the curing process. For example, moulding compounds are made with cut fibres which are placed in a random pattern, and the resulting physical properties are the same in all directions. Polyester and vinyl ester resins are cured by the addition of a variety of catalysts, promoters, accelerators and inhibitors, all of which may be mixed into the resin in small accurately weighed proportions, generally in the range of 0.5 to 2.0% by weight. Small variations in prescribed amounts of additives will have little effect on the overall cure, either delaying or advancing slightly the time to gel and achievement of a full cure.
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