Publisher Summary This chapter discusses the properties of generic polymers. Polyethylene is the commonest of thermoplastics. They are often described as linear polymers that are the chains and are not cross-linked. That is why they soften if the polymer is heated, and the secondary bonds that bind the molecules to each other melt so that it flows like a viscous liquid, allowing it to be formed. The molecules in linear polymers have a range of molecular weights, and they pack together in a variety of configurations. Some, like polystyrene, are amorphous and others, like polyethylene, are partly crystalline. This range of molecular weights and packing geometries means that thermoplastics do not have a sharp melting point. Instead, their viscosity falls over a range of temperature like that of an inorganic glass. Thermoplastics are made by adding together subunits to form long chains. Many of them are made of the unit. Epoxy, familiar as an adhesive and as the matrix of fiber glass, is a thermoset. Thermosets are made by mixing two components that react and harden either at room temperature or on heating.