Polyamides mechanical degradation

Mechanical degradation cannot be stabilized. Mechanical fatigue or internal strains imposed on the polymer mass by processing done under incorrect conditions decrease its resistance to corrosion. PE, PS, PMMA, rigid PVC and polyamides are sensitive to the formation of internal strains. If one of these... 

This polyamide exhibits a reverse degradation-polymerization reaction [2088]. Under UV irradiation at 224 nm, the polyamide is degraded and characteristic trans-cinnamide absorption is observed. This trans-cinnamide absorption disappears following exposure to light of wavelength 304 nm (Fig. 4.9), and poly(hexamethylene-a-truxillamide) is reformed (Fig. 4.10). The observed reverse reaction can be explained by the following mechanism ... 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

Braun, U., Schartel, B., Fichera, M.A., and Jager, C. 2007. Flame retardancy mechanism of aluminum phosphinate in combination with melamine polyphosphate and zinc borate in glass-fiber reinforced polyamide 6,6. Polym. Degradation Stab., 92, 1528-1545. 

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

Pokholok T. V., Gaponova 1. S., Davydov E. Ya., Pariiskii G. B. Mechanism of stable radical generation in aromatie polyamides on exposure to nitrogen dioxide. Polym. Degrad. Stab. 2006 91 (10) 2423-28. 

Hydrophobic interaction chromatography Mode of chromatography using as a retention mechanism the molecular interactions between the hydrophobic or nonpolar regions of the external surface of protein molecules and chains bonded to a siUca support. Although C18 alkyl chains would qualify, the name is reserved to polyether or polyamide chains which cause little degradation of the purified proteins. 

Gijsman, P. Steenbakkers, R. Furst, C. Kersjes, J. Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66. Polym. Degrad. Stab. 2002, 78,219-224. 

Studies of polyamides " also illustrate mechanisms involving MglOlD. Mg(OH)2 decomposes before the decomposition of PA-6 and after the decomposition of PA-66. The water formed by Mg(OH)2 before the decomposition of PA-6 gives a fire retardant effect. However, it is possible that the water thus formed may hasten the degradation of the polymer by hydrolyzing it. 

Effects of maleated ethylene-propylene diene rubber (EPDM) on the thermal stability of pure polyamides, and polyamide/EPDM and polyamide/poly (ethylene terephthalate) blends kinetic parameters and reaction mechanism/Zl. Vieira, V.L. Severgnini, D.J. Maasra MS. Soldi, E.A Pinheiro, AT.N. Pires and V. ScAAilPolymer degradation and stability (2001), 74 1 151-157. 

By a similar degradation and graft copolymer formation mechanism block copolymers could be obtained by reactive extrusion. A prerequisite is the presence of functional groups and the interaction of these groups with immiscible polymers during melt processing. Several polyamide block copolymers are synthesised by a melt-modification process. 

The use of modified PTFE micropowder in combination with the mechanism of the controlled degradation of polyamide and the utilisation of melt-mixing devices like a lab melt mixer or twin-screw extruders are the basis of the reactive compa-tibilisation of the completely immiscible polymers. On the basis of the discussed mechanism PTFE polyamide block copolymers are formed by reactive extrusion ... 

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