Aromatic polyamides degradation mechanisms

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). 

To the category of amorphous, aromatic polyamides (PARA) belong polyphthalamides (PPhA), e.g., AmodeF (Tg = 127 °C, T = 310 C, HDT = 285 °C, CUT = 180 °C). PPS/PARA blends were formulated to increase the reinforcing effects of GF on PPS. They show good processability, mechanical performance, and resistance to thermal degradation. 

The thermo-oxidative cross-linking of nylon 6 and the resistance to oxidation of its graft copolymers with polyacrylonitrile have been studied. The thermal and thermal oxidative degradation of poly(/w-phenylene isophthalamide) and other aromatic polyamide compositions, together with comparative studies on the mechanisms of degradation of poly(p-benzamide) and poIy(p-phenylene terephalamide) have all been reported. 

The proposed mechanism requires a tautomeric shift which would be prevented if the -NH group was converted to the -NCH3, We have prepared such polyamides and it appeared that the benzoxazole did not form.Studies have continued in comparing F, Cl and Br substituents and appropriate dihalo pol3rmers. Additional studies have been done on the effects of electron withdrawing substituents on the thermal degradation of aromatic polyamides... 

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