Mechanical and Thermal Degradation

Thermal stresses can further accelerate both the chemical and mechanical degradation mechanisms. 

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The major thrust of the stress MS studies to date has focused on gathering evidence to support the kinetic theory of fracture proposed by Zhurkov. For the Zhurkov kinetic theory of fracture to be applicable to the material in question, not only must the mechanical and thermal degradation products be identical, but both processes must follow the same degradation mechanism. 

These materials, primarily aluminized polymers, may be very sensitive to mechanical and thermal degradation by radiation. Heating due to eddy currents induced by pulsed magnetic fields may be significant. 

These two examples demonstrate both the value of FTIR-EGA for studying both mechanical and thermal degradation and the potential for erroneous results arising from the effects of mechanical degradation on subsequent polymer degradation studies. 

Calcium hypochlorite is isolated from solution as mixed crystal whose composition depends on the purity of the lime and on process conditions. It is recovered by filtration and dried with hot air to give a sohd containing typically 65-70% available chlorine. Careful handling and process control minimize mechanical and thermal degradation of the product. 

On a duct of cylindrical geometry the overall degradation of the molten PET was obtained as the sum of mechanical and thermal degradation in relation to the diameter of the duct [757]. 

Poly(phenylene oxide) (PPO) is a thermoplastic, linear, noncrystalline polyether commercially produced by the oxidative polymerization of 2,6-dimethylphenol in the presence of a copper-amine catalyst. PPO has become one of the most important engineering plastics widely used for a broad range of applications due to its unique combination of mechanical properties, low moisture absorption, excellent electrical insulation property, dimension stability and inherent flame resistance. This chapter describes the recent development of this polymer, particularly on the production, application, compounding, properties of its alloys and their general process conditions. The polymerization mechanism and thermal degradation pathways are reviewed and new potential applications driven by the increasing environmental concerns in battery industry, gas permeability and proton-conducting membranes are discussed. 

There is also a significant difference between degradation products after mechanical and thermal degradation found in certain experiments [1,48] (Fig. 3.16). 

Two major mechanisms for thermal degradation and one minor mechanism are shown in Fig. 9. The first mechanism is the reverse of urethane formation. The second mechanism, which was proposed by Fabris, forms a primary amine and an olefin. It involves a six-member intermediate, as shown in Fig. 10. A thermal... 

For cross-ply laminates, a knee in the load-deformation cun/e occurs after the mechanical and thermal interactions between layers uncouple because of failure (which might be only degradation, not necessarily fracture) of a lamina. The mechanical interactions are caused by Poisson effects and/or shear-extension coupling. The thermal interactions are caused by different coefficients of thermal expansion in different layers because of different angular orientations of the layers (even though the orthotropic materials in each lamina are the same). The interactions are disrupted if the layers in a laminate separate. 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

The addition of heat shifts the equilibrium concentrations away from the products and back towards the reactants, the monomers. This is one reason why processing these types of polymers is often more difficult than processing products of chain growth mechanisms. The thermal degradation process can be dramatically accelerated by the presence of the low molecular weight condensation products such as water. Polyester, as an example, can depolymerize rapidly if processed in the presence of absorbed or entrained water. 

Liu Q, Zou Y, Bei Y, Qi G, Meng Y (2008) Mechanic properties and thermal degradation kinetics of terpolymer poly (propylene cyclohexene carbonates). Mater Lett 62 3294—3296... 

G. N. Richards and F. Shafizadeh, Mechanism of thermal degradation of sucrose. A preliminary study, Aust. J. Chem., 31 (1978) 1825-1832. 

Macromolecular structures such as star copolymers have been synthesized in the search for polymers with new mechanical and thermal properties and new degradation profiles. Fig. 7 shows a schematic representation of four-armed homo- and block copolymers. 

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