Degradation of epoxy-clay nanocomposites after UV exposure and moisture attack
by Woo Shui Cheung
M.Phil. Mechanical Engineering
xviii, 113 leaves : ill. ; 30 cm
The degradation behavior and residual properties of epoxy-clay nanocomposites after being exposed to moisture and UV were studied. The nanocomposite was fabricated by in-situ polymerization after high-speed shear mixing and ultrasonication of resin-organoclay mixture. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that the I.30P system (nanocomposite with I.30P clay) exhibited a large increase in intergallery distance from 2.24 nm to over 8 nm during the curing process, which resulted in a mixture of intercalation and exfoliation. Differential scanning calorimetry (DSC) indicates the catalytic effect of organic modifier on curing reaction. The absence of an exothermic peak also confirmed the fully cured state by using a modified curing profile. Dynamic mechanica...[ Read more ]
The degradation behavior and residual properties of epoxy-clay nanocomposites after being exposed to moisture and UV were studied. The nanocomposite was fabricated by in-situ polymerization after high-speed shear mixing and ultrasonication of resin-organoclay mixture. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that the I.30P system (nanocomposite with I.30P clay) exhibited a large increase in intergallery distance from 2.24 nm to over 8 nm during the curing process, which resulted in a mixture of intercalation and exfoliation. Differential scanning calorimetry (DSC) indicates the catalytic effect of organic modifier on curing reaction. The absence of an exothermic peak also confirmed the fully cured state by using a modified curing profile. Dynamic mechanical analysis (DMA) reveals an improvement in storage modulus at room temperature for nanocomposites. The glass transition temperature (Tg) was, however, marginally reduced with clay loading because of the disentanglement of polymer chains around the silicate surface, and a shift in stoichiometry due to epoxy homopolymerization within the clay galleries.
The moisture absorption behaviors were different depending on the type of organoclay: the behavior was similar for the neat epoxy and the KH-MT system, which could be described by Fick’s second law, whereas the Cloisite 20A and I.30P systems followed a non-Fickian fit. The deviation from Fickian diffusion was attributed to the higher intergallery distance and more uniform distribution of organoclays, which in turn allowed a longer diffusion path of water molecules in the nanocomposite. The moisture diffusivity of nanocomposites decreased with increasing clay loading for all organoclays, and it was much lower for the Cloisite 20A and I.30P systems than the KH-MT system. The corresponding moisture permeability was lower for nanocomposites containing I.30P (86 %), Cloisite 20A (66 %) and KH-MT (57 %) organoclays. It shows a systematic decrease with increasing clay loading, which agrees well with the prediction based on the simple tortuous path model. An increase in the effective penetration path due to the very high aspect ratio of the silicate layers was responsible for the reduced moisture permeability.
The neat epoxy and nanocomposites after UV exposure at varying durations were characterized to evaluate the changes in chemical composition, topography and color. Scanning electron microscopy (SEM) examination revealed that surface damage has led to microcracking after about 300 hr of UV exposure. Wider and shallower cracks with a lower degree of discoloration were presented in nanocomposites, due to the diffusion barrier properties of organoclay with a high aspect ratio. However, the presence of unexchangeable transition metal ions such as Fe2+ and Fe3+ along with low-molecular-weight organic modifiers applied to the clay accelerated the degradation of polymer, counterbalancing the ameliorating effects brought about by the barrier properties of clay. Fourier transform infra-red spectroscopy (FTIR) analysis indicated that photo-degradation generated carbonyl groups (C=O) by chain scission and the rate was slightly higher for nanocomposites than neat epoxy. While moisture further accelerated the photo-degradation process through the enhanced mobility of free radicals and ions, the organoclay could limit the deteriorating effects of moisture, resulting in a much better overall resistance to photo-degradation in the presence of moisture for nanocomposite than neat epoxy.
Residual mechanical properties of neat epoxy and nanocomposites were evaluated after they were exposed to moisture and UV. The Tg for both nanocomposite and neat epoxy showed a linear decrease with increasing moisture content due to the plasticization effect of moisture. Flexural modulus was increased by 7-8 % by the addition of 5 wt% I.30P. Although the modulus gradually dropped at a decreasing rate with moisture exposure durations, the nanocomposite in the wet condition still sustained a similar modulus level as the neat epoxy in the dry state. Tensile modulus was increased with the incorporation of clay, at the expense of a reduced strength and failure strain, which may be caused by the presence of clay aggregates, voids and different cure kinetics induced by organoclay. Failure strain was largely reduced for neat epoxy after UV exposure and the effect was diminished with clay loading. Microhardness test further revealed the embrittlement effect with increased hardness values along the UV exposure durations. A heavier embrittlement resulting in over a two fold increase in hardness was found in neat epoxy after 2000 hr of exposure, as compared to 5 wt% I.30P nanocomposite. Modulus changes from the exposed surface into the depth of material as well as the thickness of the degraded layer at different UV exposure times were determined by nanoindentation technique. The modulus decreased systematically from the exposed surface into the depth of material, followed by a steady state which indicated the property of unaffected bulk material. With the addition of 5 wt% I.30P, the modulus profile has a steeper initial slope with the steady state reached at a smaller depth. The thickness of the degraded layer increased gradually with exposure time and it was much smaller for 5 wt% I.30P nanocomposite than neat epoxy (29.81 μ m versus 39.15 μ m at 2000 hr of exposure). This was due to the restricted diffusion of radicals and oxygen causing photo-degradation and explained the wider but shallower crack morphology after UV exposure. Fracture toughness calculated from the radial crack measurements after microhardness test was found to be 20 % higher for nanocomposite than neat epoxy. The toughness was one order of magnitude lower after UV exposure, which was caused by the embrittlement effect of UV as well as overestimation of the crack lengths on the surface after UV exposure.