Evaluation of Photodegradation of Nylon 6 Studied by Dynamic Viscoelastic Measurements

Abstract

Photodegradation of nylon 6 was studied by dynamic viscoelastic measurements. Dynamic viscoelastic properties of weathering nylon 6 were also measured. In order to investigate the effect of wavelength of the irradiation, spectrally dispersed ultraviolet light between 219 and 415nm from 2kW xenon lamp were irradiated on nylon 6 film and changes in the dynamic modulus E′ were measured at each avelength. The values of E′ were increased by the irradiation at wavelength below about 300nm. From the results along with those X-ray diffractometry, viscosity measurements, and solubility tests in m-cresol, it was suggested that the increase in E′ resulted from the crosslinking reaction. Variation of E′ during irradiation at 253.7nm was also measured using a low pressure mercury lamp. It was found that E′ decreased rapidly in the region of short times reflecting the occurence of scission reactions, then increased at longer times, and reached a certain equilibrium value. The increase of E′ at longer times was interpreted as due to the crosslinking reactions. The change in E′ with time was well expressed by the following equation: E_t′=E₀′exp(-k₁t)+E′_∞[1-exp(-k₂t)], where E_t′ is the E at time t, E₀′ is that at t=0, E′∞is the equilibrium value of the E′, and k₁ and k₂ are the rate constants.
Changes of E′ and density during weathering at Hokkaido and Choshi were measured. Both E′ and density were increased with weathering time. The changes in X-ray diffraction patterns have revealed the result of the transition of the crystal structure of nylon 6 from the β form to the α form, which was caused by absorption of water coming from rainfall. The values of [η] decreased steadily with exposure time. It was inferred that these were caused by a scission reaction due to the sunlight, a kind of low energy ultraviolet rays with wavelength longer than about 300nm. In the loss tangent versus temperature plots of the weathering nylon 6, α, β, γ relaxations were observed. Also an additional dispersion was found at about 30°C, which was in between the α and β relaxations. The dispersion was assigned to the shifted α relaxation due to plasticizing effect of absorbed water. In the temperature range between -20 and -120°C, E′ of weathering sample was higher than that of dry nylon 6 (α form). This was also inferred as related to the absorbed water. However, E′ of dry nylon 6 of α structure was higher than that of weathering sample at temperatures above about 20°C.