Aging of Polyamide 11 nanocomposites

Thermal and photo-oxidative degradation processes occurring on PA11 nanocomposites (NC) with Cloisite®30B were investigated to establish the eventual contribution of organo nanoclay modifier on degradation mechanisms. It is well known that polyamides are not intrinsically stable in the presence of oxygen and/or humidity especially at high temperature. The literature identified as most relevant the ?-CH hydrogen abstraction oxidative mechanism followed by the Karsten and Rossbach pathway for the interpretation of residue formation. Contradictory opinions about the degradation processes involving PAs NC materials were found. Furthermore, was not clear as nanoclays affect the degradation pathways of PAs based NC. Thermal-oxidation of PA11NC was performed in a glass vessel at 215°C under flow or atmospheric air up to 150 minutes, whereas the photo-aging was carried out on QUV apparatus at 60°C using UV lamps (340nm) up to 7 days. The oxidized materials were analyzed by size exclusion chromatography (SEC), viscometry and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Data obtained for thermo-oxidised PA11 NC, ascertained an active contribution of organo-modifier clays in accelerating cross-linking reactions. PA 11 NC samples degraded faster than virgin PA11 material producing a higher amount of insoluble residue. The formation of gel fraction was accomplished by the appearance of a characteristic peak in the MALDI spectra. We suppose that ? olefins produced by Hofmann elimination reaction from Cloisite®30B were responsible for the increase of rate of cross linking reactions. In fact, ? olefins are very susceptible to air and moisture, producing oxygenated (aldehydes, acids) as final products. During initial step of auto-oxidation, the formation of ? olefin hydroperoxides is accomplished by the capture of available hydrogens; the favourable attack is without doubt to the ? amino methylene positions of PA11 chains. At this point, the well known ?-CH hydrogen abstraction mechanism proceeds, accelerating the formation of amide terminal chain groups and consequently the gel fraction. The amide degradation products were identified and revealed as a characteristic peak in the MALDI spectra. Data obtained from photo-oxidation of PA11 nanocomposites suggested that organo-modifier clays do not significantly contribute to modify photoreaction pathways of polymeric material. Differently from PA6 and PA66, MALDI peaks deriving from NORRISH photo-cleavage were absent from the PA11 spectra collected at different exposure time. The only mechanism active during photo-exposure of PA11 samples appears to be the ?-CH hydrogen abstraction.