Compare thermoplastic and thermosetting polymers (a) on the basis of mechanical characteristics upon heating and (b) according to possible molecular structures.

Understanding the mechanical characteristics of polymers is pivotal for engineering applications. Two main polymer groups, thermoplastics and thermosetting polymers, reveal different behavioral patterns upon heating due to distinct molecular structures.

Thermoplastics

Unique due to their ability to soften when heated and solidify when cooled, thermoplastics like polyethylene and PVC are excellent for applications requiring multiple rounds of molding. As temperature rises, their ductility increases, and rigidity decreases, which means they can be reshaped via techniques such as injection molding.

Thermosets

Contrastingly, thermosetting polymers are structurally solid and maintain rigidity due to strong cross-links. This makes them ideal for high-temperature applications, where other materials might fail. Heating thermosets does not lead to the same pliability as seen in thermoplastics; instead, they retain their shape and resist deformation up to their degradation point.

The polymer molecular structures determine the materials' overall properties and are a deciding factor in their mechanical characteristics.

Structure of Thermoplastics

Thermoplastics are typically composed of linear or branched chains held together by non-covalent bonds, allowing mobility within the chains upon heating. As a result, these materials can be easily reused and recycled, aligning with eco-friendly practices.

Structure of Thermosets

The intricate network of covalently bonded cross-links in thermosets is what imparts strength and heat resistance. This permanence in the bond structure also means once molded; they cannot be reprocessed or remolded, limiting their recyclability but enhancing their suitability for durable end-use products.

The heat treatment of polymers is a sophisticated process that renders the final attributes of the product. Through this treatment, thermoplastics can be molded, shaped, and welded for a variety of uses. The ideal process involves controlled heating cycles for softening and shaping, followed by cooling to solidify the desired form.

Heat can be an adversary for thermosetting polymers once they've been set. Exposure to heat beyond a thermoset's tolerance can cause irreversible damage or decomposition. Thus, proper heat management is critical, and for thermosetting items, the initial heat treatment is all-important, as it defines the final permanence of the object.

Different polymer processing methods are chosen based on the desired end product and the type of polymer used.

Thermoplastic Processing

A variety of methods like extrusion, which pushes softened material through a die, to complete molding processes are utilized for thermoplastics. These methods allow for high production efficiency and design versatility.

Thermoset Processing

Thermosetting polymers often undergo processes such as compression molding or resin transfer molding. Positioned within molds and then heated, thermosets solidify into their shape — a process that's irreversible due to the cross-linking but provides products with superior mechanical strengths.