Degradation thermosets

Polyolefins in fact lend themselves to this straightforward recycling procedure. Other thermoplastics like polyamides and polyesters need more careful handling because of their sensitivity to moisture absorption and molecular degradation. Thermosets are, by definition, not recyclable in the true sense however by comminuting and recompounding with fresh monomers, most thermoset polymers can in effect be recycled without significant loss of performance. 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

Thermosets and thermoplastics behave differently from each other in fires. Thermosets do not melt when heated but may well undergo further crosslinking. The presence of such additional crosslinks hinders movement of any volatile degradation products through the polymer matrix. Hence the combustion zone tends to be starved of fuel and for this reason thermosets tend to be relatively non-flammable. 

The hydrogenolytic degradation of thermosets by partially hydrogenated aromatics such as tetraline and 9,10-... 

Some polymers like PE and NR get cross-linked on exposure to radiation while others like those based on vinylidene polymers, e.g., polymethylmethacrylate (PMMA), polyisobutylene, degrade. Certain other types of polymer stmctures (high aromatic content or thermoset) resist degradation by high-energy radiation. Coating polymers usually contain acrylic, methacryUc, or fumaric vinyl unsaturation along or attached to the backbone. 

Accelerate chemical, photochemical, biochemical reactions or processes, e.g. cross-linking or degradation of polymers. Also called promoters, co-catalysts. Refer usually to the cure process in thermosetting resins. 

Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. Instead, when exposed to high temperatures, thermosets degrade into char. Examples of thermosets include some polyurethanes and epoxy resins. 

The term charring refers to the complete degradation of a polymer after which there is no longer any polymeric character to observe. Charring results from chain scission reactions that are left unchecked and is the typical process by which thermosets degrade. The resulting material is typically black and brittle. 

Literally removal , but applied particularly in space technology to the process of using up the frictional heat developed on re-entry of the vehicle into the Earth s atmosphere by degradation of the heat shield. Certain thermoplastics, thermosetting resins and polytetrafluoroethylene have been evaluated as ablative materials. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Infusibility thermosets are degraded by heat without passing through the liquid state. This improves some aspects of fire behaviour except for particular cases, they do not drip during a fire and a certain residual physical cohesion provides a barrier effect. 

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