Polymer clay nanocomposites appear poised to become an attractive new class of engineering materials. Polyethylene is the most widely used polymer in engineering products. Adding nanoclay to polyethylene can greatly enhance the mechanical properties; however, the effect on the long-term performance is less known. Polyethylene is susceptible to oxidation which is minimized by adding antioxidants. Once antioxidants are completely consumed, oxidation degradation begins and subsequently leads to material deterioration. The depletion mechanism of antioxidants in polyethylene nanoclay composites will be assessed based on thermo-oxidation in dry and wet environments. The non-oxidization behavior of antioxidants in the nanocomposite will be characterized by identifying their physical adsorption onto the surface of nanoclay as well as their hydrolysis sensitivity. The diffusion of oxygen into the nanocomposite and reaction with antioxidants will be determined and modeled to obtain the intrinsic oxidation rate constant. The critical microscopic properties that could serve as precursors for the onset of macroscopic property change will be established.
Successful completion of this project will have broader impacts on engineering materials and will strengthen graduate and undergraduate education. By employing chemistry and materials testing approaches, this project will facilitate multidisciplinary collaboration and provide a platform for graduate and undergraduate students to work together in materials science and engineering disciplines. This project will also serve as a vehicle for introducing innovative new materials to civil engineering students. The research findings can be beneficial to many civil engineering sectors where polyethylene products are used as essential components of the system.
The main outcome of this project is an improved understanding of the ways that clay particles reduce the effectiveness of antioxidants in plastics. Antioxidants are commonly added to plastics such as polyethylene to inhibit degradation of plastics due to oxidation. In polyethylene phenolic antioxidants are very effective; however when clay is blended into polyethylene to make a nanocomposite, phenolic antioxidants deplete much more rapidly. In this project nanocomposite samples were prepared with a range of compositions of antioxidants and clay, the samples were aged at high temperatures to accelerate degradation, and samples were sectioned to determine spatial distributions of antioxidant depletion during over one year of aging. Experimental measurements of antioxidant depletion are shown in Images 1, 4, and 6. Predictions of antioxidant depletion from mathematical models are shown in Images 4, 5, and 6 which clarify the mechanisms of antioxidant depletion. Clay platelets hinder movement of antioxidants within the nanocomposite by creating a tortuous pathway for antioxidant diffusion. Image 2 displays X-ray data showing differences in orientation of clay nanoparticles between the surface and center of a polyethylene nanocomposite sample. This gradient in orientation leads to formation of a skin near the surface of the samples in which antioxidants deplete more rapidly as shown in Image 1. The experiments reveal a ‘table-top’ profile where the antioxidant concentration in the core of the sample is fairly high and uniform but there is a layer near the surface that is mostly depleted of antioxidants. The formation of the depleted skin layer has been related to a region in which the clay particles are highly oriented during sample preparation; removal of the oriented layer leads to more uniform depletion. A mathematical model of polyethylene degradation that includes the stabilizing effect of antioxidants was developed and is depicted in Image 3. The mathematical model accurately predicts the experimental data as shown in Images 4 and 6 and enables deeper understanding of the mechanisms of antioxidant depletion in nanocomposites. In polyethylene containing antioxidants, the concentration of free radical species are driven to zero by stabilization reactions with antioxidants, as shown in Image 5, which produce hydroperoxides. However, in nanocomposites, decomposition of hydroperoxides is faster than in neat polyethylene leading to a cyclic reaction that depletes antioxidants. This accelerated decomposition of hydroperoxides is assumed to be caused by catalytic action of transition metal impurities in the clay. Image 4 shows that the kinetic model accurately predicts depletion of antioxidants in both neat polyethylene and nanocomposites. Image 6 shows that model predictions of antioxidant diffusion and reaction qualitatively reproduce the table-top profiles observed experimentally. The results from this project are useful for guiding development of improved antioxidant formulations for polymer nanocomposites with enhanced durability.
