Investigation and development of gas transport properties of extruded plastic products

Polypropylene pipes, produced by plastic extrusion, are widely used in both home plumbing systems and industrial applications. The oxygen transport from surrounding air, in home or industrial zone, to the inner volume of the pipe, causes corrosion on metallic equipments installed on the plumbing systems.Gas transport rate through polymeric media is governed by gas diffusion coefficient. There are several equipments to measure gas diffusion coefficient of polymer films and sheets in Cartesian coordinates. To the best of our knowledge there is no commercial equipment and published research work to measure gas transport properties of plastic pipes with using constant volume - variable pressure method in cylindrical coordinates. Due to the effect of processing history on the polymer morphology, measurement of the diffusion coefficient of polypropylene thin films and sheets produced with film or sheet extrusion in Cartesian coordinates can be different from the diffusion coefficient of the thick polypropylene pipes produced with pipe extrusion in cylindrical coordinates.In this study, a new measurement system has been designed and produced to measure gas diffusion coefficient of plastic pipes in cylindrical coordinates. Diffusion coefficients of polypropylene pipes which have 20 mm outer diameter and 3,4 mm wall thickness have been measured with this novel test system.With accelerated aging of polypropylene pipes, it was simulated to see the morphological and mechanical changes on plastic pipes during long term real working conditions after installation on home plumbing or industrial systems.The effect of accelerated thermal and mechanical aging on the polypropylene pipes have been determined by Positron Annihilation Lifetime Spectroscopy (PALS), Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Melt Flow Index (MFI), density measurements and tensile testing. The effect of the accelerated aging type on the gas diffusion coefficient has also been investigated.Nanoclay - polypropylene, short glass fiber - polypropylene and calcium carbonate - polypropylene composite matrixes have been prepared by twin screw extruder and multilayer polypropylene pipes have been produced with using these composite matrixes as middle layer. The effect of multilayer structures on the diffusion coefficient of the plastic pipe has been analyzed.As a result of this work, a new test system has been developed to measure gas diffusion coefficient of the polypropylene pipes with using pristine samples. It was found that the diffusion coefficient of the polypropylene pipe is higher than the reported diffusion coefficient of the polypropylene film or sheet in the literature.It was determined that the free volume radii of the crystalline phase, stress on break and strain on break decreased; crystallinity, carbonyl and hydroxyl indexes and densities increased for all aged samples.It was found that accelerated aging in a heating oven at 110 ºC and accelerated aging with hot and cold thermal cycling between 90 ºC and 20 ºC cause an increase in oxygen diffusion coefficient of tested polypropylene pipes, but accelerated aging with hydrostatic internal pressure test at 95 ºC and 35 bar causes a decrease in the oxygen diffusion coefficient of tested polypropylene pipes.All tested multilayer pipes with nanoclay, calcium carbonate and glass fiber middle layers have showed a decrease in oxygen diffusion coefficient. Nanoclay composite layer showed a decrease in diffusion coefficient three times more than other inorganic composite structures even nanoclay loading in the matrix was five times lower than others.