Polymers Large molecules thermosetting

Chcmically, Bakelite is a phenolic resin, produced by reaction of phenol and formaldehyde. On heating, water is eliminated, many cross-links form, and the polymer sets into a rocklike mass. The cross-linking in Bakelite and other thermosetting resins is three-dimensional and is so extensive that we can t really speak of polymer "chains." A piece of Bakelite is essentially one large molecule. 

This reaction involves reacting different monomers to form polymers and large molecules with the elimination of, e.g. water or HC1, in the process. The same rules apply as for the addition process and the polymers formed by this process can be used to prepare both thermoplastic and thermosetting plastics. Table 4.2 is a list of common polymers prepared by the condensation process. 

All polymers are formed by the creation of chemical linkages between relatively small molecules, or monomers, to form very large molecules, or polymers. As mentioned, if the chemical linkages form a rigid, cross-linked molecular structure, a thermosetting plastic results. If a somewhat flexible molecular structure is formed, either linear or branched, a thermoplastic results. Illustrations of these molecular structures are presented in Fig. IN.2. 

For the most part, plastics are man-made since very few plcistlcs are natural, i.e.- nature-made. Natural plastics include large molecular-wei t proteins and similar molecules. Man-made plastics can be classified as either thermoplastic or thermosetting. Each class derives its physical properties from the effects of application of heat, the former becoming "plastic" (that is- it becomes soft and tends to flow) while the latter becomes less "plastic" and tends to remain in a softened state. This difference in change of state derives from the actual nature of the chemical bonds in the polymer. Thermoplastic polymers generally consist of molecules composed of many monomeric units. A good example is that of polyethylene where the monomeric unit is -(CH2-CH2)-. The molecule is linear... 

If a large number of branches exist that connect all of the backbone molecules into a three-dimensional network, the material will not flow when heated, and it is considered a thermoset resin. Vulcanized rubber is an example where the sulfur linkages create a three-dimensional network, converting the precursor rubber into a solid thermoset material. Crosslinked backbone chains are shown in Fig. 2.8(e). When extruding many thermoplastics, the polymer can undergo chemical reactions to form small amounts of crosslinked material. Partial crosslinking is a problem with some PE resins that contain residual double bonds that are made using... 

The extreme hardness and lack of formability of thermoset plastics is due to the extremely large number of primary valence bonds between the plastic s atoms. In an extreme case of crosslinking with covalent bonds practically no secondary valence bonds exist that can be loosened by increasing the temperature. This means that no thermoplastic processing of the polymer is possible. A thermoset plastic can be depicted as being a huge single molecule. The removal of these intra molecular bonds by increased tempera-... 

Polymers containing heterocycles in the backbone include a variety of compounds, as the diversity of heterocyclic molecules is quite large. The polymers from this class may contain groups derived from furan, thiophene, pyrrole, isoindole, benzimidazole, benzothiazole, benzoxazole, quinoxaline, etc. Macromolecules with a ladder backbone containing, for example, a phenoxazine unit in their structure also are known. Amino thermosetting resins from melamine can be considered as polymers containing heterocycles in their structure. 

Polymers are the backbone of what we call plastics. They are chemical compounds that have very large, molecular, chainlike structures. Plastics are often classified into two categories thermoplastics and thermosets. When heated to certain temperatures, the thermoplastics can be molded and remolded. For example, when you recycle Styrofoam dishes, they can be heated and reshaped into cups or bowls or other shapes. By contrast, thermosets can not be remolded into other shapes by heating. The application of heat to thermosets does not soften the material for remolding instead, the material will simply break down. There are many odier ways of classifying plastics for instamce, they may be classified on the basis of their chemical composition, or molecular structure, or the way molecules aue airranged, or their densities. For example, based... 

A polymer is a material composed of large macromolecules. These macromolecules are formed by chains of hundreds or thousands of coimected (polymerized) monomer molecules. The three main classes of polymers are thermoplastics, elastomers, and thermosets. They differ in the degree of cross-linking of their macromolecules - from no cross-linking (thermoplastics) to moderate cross-linking (elastomers, rubbers) to high cross-linking (thermosets). 

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