Polymers are substances containing a large number of structural units joined by the same type of linkage. These substances often form into a chain-like structure. Polymers in the natural world have been around since the beginning of time. Starch, cellulose, and rubber all possess polymeric properties. Man-made polymers have been studied since 1832. Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined.
Polymers already have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.
Plastics are synthetic materials called polymers, which are long-chain molecules made up of repeating units joined together. These units contain various combinations of oxygen, hydrogen, nitrogen, carbon, silicon, chlorine, fluorine, and sulfur. Although plastics are soft and moldable and approach a liquid condition during manufacture, they are solid in their finished state. As more repeating units are added, the plastic’s molecular weight increases. Addition of more repeating units to the chain makes the molecule heavier.
The mechanical and physical properties of plastics are directly related to the bonds between molecular chains, as well as to the chain length and composition. Plastic properties can also be modified both by alloying and blending with various substances and reinforcements.
CLASSIFICATION
The classification of plastics can be extensive and confusing. However, two major groups can be identified: Thermoplastics and Thermosets. In addition to the broad categories of thermoplastics and thermosets, polymers can be classified in terms of their structure, i.e., crystalline, amorphous, and liquid crystalline. Other classes of plastics commonly referred are copolymers, alloys, and elastomers. Finally, additives, reinforcements, and fillers play a major role in modifying properties.
THERMOPLASTICS
Thermoplastics are resins that repeatedly soften when heated and harden when cooled. Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening temperatures vary with polymer type and grade. Because of thermoplastics’ heat sensitivity, care must be taken to avoid degrading, decomposing, or igniting the material. Nylon, acrylic, acetal, polystyrene, polyvinyl chloride, polyethylene, and cellulose acetate are just a few examples of the many rigid thermoplastic resins currently available. Also within this group are highly elastic, flexible resins known as thermoplastic elastomers (TPEs).
Most thermoplastic molecular chains can be thought of as independent, intertwined strings resembling spaghetti. When heated, the individual chains slip, causing plastic flow. When cooled, the chains of atoms and molecules are once again held firmly. When subsequently heated, the chains slip again. There are practical limitations to the number of heating/cooling cycles to which thermoplastics can be subjected before appearance and mechanical properties are affected.
THERMOSETS
Thermosets are plastics that undergo chemical change during processing to become permanently insoluble and infusible. Phenolic, amino, epoxy, and unsaturated polyester resins are typical thermoset plastics. Natural and synthetic rubbers such as latex, nitrile, millable polyurethane, silicone, butyl, and neoprene, which attain their properties through a process known as vulcanization, are also thermoset polymers. The structure of thermoset plastics is also chainlike and, prior to molding, very similar to thermoplastics.
However, cross-linking is the principal difference between thermoset and thermoplastic systems.
When thermosets are cured or hardened, crosslinks are formed between adjacent molecules, resulting in a complex, interconnected network. These cross bonds prevent the individual chains from slipping, thus preventing plastic flow when heat is added. If excessive heat is added to the thermoset resin after the cross-linking is complete, the polymer is degraded rather than melted. This behavior is somewhat similar to an egg when it is cooked; further heating does not return the egg to its liquid state, it only burns.
Polymers already have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.
Plastics are synthetic materials called polymers, which are long-chain molecules made up of repeating units joined together. These units contain various combinations of oxygen, hydrogen, nitrogen, carbon, silicon, chlorine, fluorine, and sulfur. Although plastics are soft and moldable and approach a liquid condition during manufacture, they are solid in their finished state. As more repeating units are added, the plastic’s molecular weight increases. Addition of more repeating units to the chain makes the molecule heavier.
The mechanical and physical properties of plastics are directly related to the bonds between molecular chains, as well as to the chain length and composition. Plastic properties can also be modified both by alloying and blending with various substances and reinforcements.
CLASSIFICATION
The classification of plastics can be extensive and confusing. However, two major groups can be identified: Thermoplastics and Thermosets. In addition to the broad categories of thermoplastics and thermosets, polymers can be classified in terms of their structure, i.e., crystalline, amorphous, and liquid crystalline. Other classes of plastics commonly referred are copolymers, alloys, and elastomers. Finally, additives, reinforcements, and fillers play a major role in modifying properties.
THERMOPLASTICS
Thermoplastics are resins that repeatedly soften when heated and harden when cooled. Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening temperatures vary with polymer type and grade. Because of thermoplastics’ heat sensitivity, care must be taken to avoid degrading, decomposing, or igniting the material. Nylon, acrylic, acetal, polystyrene, polyvinyl chloride, polyethylene, and cellulose acetate are just a few examples of the many rigid thermoplastic resins currently available. Also within this group are highly elastic, flexible resins known as thermoplastic elastomers (TPEs).
Most thermoplastic molecular chains can be thought of as independent, intertwined strings resembling spaghetti. When heated, the individual chains slip, causing plastic flow. When cooled, the chains of atoms and molecules are once again held firmly. When subsequently heated, the chains slip again. There are practical limitations to the number of heating/cooling cycles to which thermoplastics can be subjected before appearance and mechanical properties are affected.
THERMOSETS
Thermosets are plastics that undergo chemical change during processing to become permanently insoluble and infusible. Phenolic, amino, epoxy, and unsaturated polyester resins are typical thermoset plastics. Natural and synthetic rubbers such as latex, nitrile, millable polyurethane, silicone, butyl, and neoprene, which attain their properties through a process known as vulcanization, are also thermoset polymers. The structure of thermoset plastics is also chainlike and, prior to molding, very similar to thermoplastics.
However, cross-linking is the principal difference between thermoset and thermoplastic systems.
When thermosets are cured or hardened, crosslinks are formed between adjacent molecules, resulting in a complex, interconnected network. These cross bonds prevent the individual chains from slipping, thus preventing plastic flow when heat is added. If excessive heat is added to the thermoset resin after the cross-linking is complete, the polymer is degraded rather than melted. This behavior is somewhat similar to an egg when it is cooked; further heating does not return the egg to its liquid state, it only burns.
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