Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites

Abstract

Nylon 6 nanocomposites based on various quaternary alkyl ammonium organoclays were prepared by melt processing using a twin screw extruder. Dilute solution viscosity techniques were used to evaluate the level of polymer molecular weight degradation experienced during nanocomposite compounding; whereas colorimeter techniques were used to document color formation. In general, a significant reduction in nylon 6 matrix molecular weight was observed, which is believed to stem, in part, from reaction(s) between the surfactant of the organoclay and the polyamide chains. The level of degradation depends on both the type of nylon 6 material used and the surfactant chemistry in the organoclay. For a given organoclay, nanocomposites based on high molecular weight nylon 6 materials experience more matrix degradation, as well as color formation, than those based on low molecular weight materials; this is believed to arise from increased exposure of the organoclay surface to the nylon 6 owing to increased platelet exfoliation. Different organoclays lead to different levels of polymer degradation and color formation, depending upon the level of unsaturation present in the organic surfactant; the higher the number of double bonds the greater the degradation and the deeper the color formation. The primary mechanism of degradation is believed to be thermo-oxidative. Melt mixing of nylon 6 with model compounds, long-chain alkenes, shows that the same mode of degradation i.e. via double bonds can be replicated. In addition to unsaturation effects, the presence of hydroxyl–ethyl groups, opposed to methyl groups, in the organoclay surfactant, results in more color. Isothermal thermogravimetric analysis (TGA) was conducted on the organoclays to determine if thermal stability was a cause of molecular weight degradation; although, this relationship does not seem to exist, a direction correlation is observed between the organoclay degradation and nanocomposite modulus, or indirectly level of exfoliation. Use of antioxidant was found to reduce the amount of molecular weight loss. All evidence suggests that morphology and physical properties of nanocomposites formed from nylon 6 are not measurably affected by the reactions that lead to molecular weight degradation or color formation.