Polymer degradation thermosets

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. 

Figure 3.5 Programmed TGA to 800 °C under nitrogen at 5 °C/min of stoichiometric thermoset resins based on DGEBA. DDM resin cured with diamino diphenyl methane, mPDA resin cured with m-phenylene diamine, pPDA resin cured with p-phenylene diamine, mXDA resin cured with weto-xyylene diamine, AN the stoichiometric thermoplastic reaction product of DGEBA and aniline (included for comparison). Reproduced with permission from T. Dyakonov, P.J. Mann, Y. Chen and W.T.K. Stevenson, Polymer Degradation and Stability, 1996,...

Poly(glycerol sebacate) poly(glycerol sebacate) (PGS) is completely bioresorbable with degradation products eliininated through natural metabolic pathways (Vert et al., 1992). This hydrophilic, partially semi-crystalline polymer has thermoset elastomeric properties that can be tuned via the modulation of curing temperamre, time and molar ratio of glycerol to sebacic acid (Aydin et al., 2012, 2013 Liu et al., 2007). 

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. 

Hdrold Sebastian. Phosphorus flame retardants in thermoset resins. Polym. Degrad. Stab. 64 no. 3 (1999) 427-431. 

TibUetti Lucie, Ferry Laurent, Longuet Claire, Mas Andre, Robin Jean-Jacques, and Lopez-Cuesta Jose-Marie. Thermal degradation and fire behavior of thermoset resins modified with phosphorus containing styrene. Polym. Degrad. Stab. 97 no. 12 (2012) 2602-2610. 

Larche, J.-F., Bussi re, P.-O., Gardette, J.-L. Photo-oxidation of acrylic-urethane thermoset networks. Relating materials properties to changes of chemical structure. Polym. Degrad. Stab. 96, 1438-1444 (2011)... 

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. 

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. 

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