Thermosets classification

Although there are numerous minor classification schemes for polymers, depending on how one wishes to categorize them, nearly all polymers can be placed in one of two major classifications—thermosetting materials (or thermosets) and thermoplastic materials. Likewise, foams, adhesives, embedding resins, elastomers, and so on, can be subdivided into thermoplastic and thermosetting classifications. 

The classification given in Table 1 is based on the process, ie, thermosetting or thermoplastic, by which polymers in general are formed into usehil articles and on the mechanical properties, ie, rigid, flexible, or mbbery, of the final product. AH commercial polymers used for molding, extmsion, etc, fit into one of these six classifications the thermoplastic elastomers are the newest. 

Polymers are often divided according to whether they can be melted and reshaped through application of heat and pressure. These materials are called thermoplastics. The second general classification comprises compounds that decompose before they can be melted or reshaped. These polymers are called thermosets. While both thermoset and thermoplastic polymers can be recycled, thermoplastic recycling is easier and more widespread because thermoplastic materials can be reshaped simply by application of heat and pressure. 

Fiber-Matrix Composites. As shown in Figure 1.75, there are two main classifications of FMCs those with continuous fiber reinforcement and those with discontinuous fiber reinforcement. Continuous-flber-reinforced composites are made from fiber rovings (bundles of twisted filaments) that have been woven into two-dimensional sheets resembling a cloth fabric. These sheets can be cut and formed to a desired shape, or preform, that is then incorporated into a composite matrix, typically a thermosetting resin such as epoxy. Metallic, ceramic, and polymeric fibers of specific compositions can all be produced in continuous fashions, and the properties of the... 

In a first classification, we can distinguish (i) boards made with an external synthetic binder, such as urea-formaldehyde or phenol-formaldehyde resins for thermosets, and (ii) boards with internal natural binders. The products cited below do not constitute an exhaustive list. 

The broadest classification for plastics is the old thermoplastic and thermosetting . Examples of the former group are polyethylene, polystyrene, and poly-(methyl methacrylate) examples of the latter are urea-formaldehyde condensation polymers, powder coatings based on polyesters, epoxy resins, and vulcanized synthetic elastomers. 

Table 1 shows a classification of thermosetting foams. Among the foams listed in this table, polyurethane foams have the largest market share in the thermosetting-plastic-foam market. 

Table 1 Classification of Thermosetting Foams Foam Reaction Property...

Since the molecular backbone can be linear, branched, or network type, this aspect is also important for polymer structure. It is known that most polymers with a linear backbone are thermoplastics, while those with network backbone are thermosetting polymers. However, for classification from a chemical point of view, this differentiation is less significant. Many polymers with linear backbone are obtained from bifunctional monomers (e.g. terephthalic acid and glycol). If the polymer is obtained from similar monomers but with more than two functionalities (e g. terephthalic acid and ethylene glycerin), the polymer will have thermosetting characteristics. For this reason, this feature is not necessarily used for a classification from the chemical point of view. 

Polyurethanes are produced by the chemical action of di-isocyanate and polyol. The properties can be varied by the type of isocyanate used and the proportion of the two monomers. There are four main groups of classification for the thermoplastic groups of polyurethane, i.e. rigid foam, flexible foam, non-cellular and cellular polymers. Two main isocyanates used are toluene di-isocyanate (TDI) and diphenylmethane diisocyanate (MDI). Polyurethanes have limited application in the pharmaceutical or medical industries. Polyurethane is used as an adhesive for laminations (thermosetting material). Like thermosetting polyurethane, thermoplastic polyurethanes can be found as esters and ethers. 

Polymers can be classified in many ways, such as by source, method of synthesis, structural shape, thermal processing behavior, and end use of polymers. Some of these classifications have already been considered in earlier sections. Thus, polymers have been classified as natural and synthetic according to source, as condensation and addition (or step and chain) according to the method of synthesis or polymerization mechanism, and as linear, branched, and network according to the structural shape of polymer molecules. According to the thermal processing behavior, polymers are classified as thermoplastics and thermosets, while according to the end use it is convenient to classify polymers as plastics, fibers, and elastomers (Rudin, 1982). 

For engineering purposes, the most useful classification of polymers is based on their thermal (thermomechanical) response. Under this scheme, polymers are classified as thermoplastics or thermosets. As the name suggests, thermoplastic polymers soften and flow under the action of heat and pressure. Upon cooling, the polymer hardens and assumes the shape of the mold (container). Thermoplastics, when compounded with appropriate ingredients, can usually withstand several of these heating and cooling cycles without suffering any structural breakdown. This behavior is similar to that of candle wax. Examples of thermoplastic polymers are polyethylene, polystyrene, and nylon. 

In the broad classification of plastics there are two generally accepted categories thermoplastic resins and thermosetting resins. 

It is not possible to discuss here the special properties of all the different types of plastic materials that can occur within these three groups. The plastics industry today, by employing copolymerization or chemical modification, is capable of producing an extraordinary number of combinations of properties, making the identification of corresponding plastics more complicated. Its physical appearance and its classification as a thermoplastic, thermoset, or elastomer therefore permit us to draw conclusions about the chemical nature of the plastic only in simple cases. But they often provide a useful additional way of characterizing the material. 

The family of plastics is classified several ways. The two major classifications are thermoplastics (TPs) and thermosets (TSs). Over 90wt% of all plastics used are TPs. The TPs and TSs in turn are classified as commodity or engineering plastics (CP and EP). Commodities such as polyethylenes (PEs), polyvinyl chlorides (PVCs), polypropylenes (PPs),... 

As technology advances, materials have been developed which evade the above classification. Mixtures of thermoplastics and thermosetting resins have been developed in which the thermoplastics material acts as a toughening agent for the thermosetting resin. Alloys or blends of two or more thermoplastics, or of plastics and rubbers, are becoming commonplace. 

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