Dimensional stability of single-site and ziegler-natta ethylene copolymers in rotational molding
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2007
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Abstract
Rotational molding, also known as rotomolding or rotational casting, is a process for manufacturing hollow seamless articles using polymer in powder form. Experiments were carried out on a bench-scale uniaxial rotational molding machine to evaluate and compare the dimensional stability of single-site and Ziegler-catalyzed ethylene copolymers in rotational molding applications. The experimental trials were conducted to determine the effect of processing conditions on the part's warpage and shrinkage. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single-site resins showed less warpage compared to the Zieglercatalyzed copolymers with otherwise comparable densities. The Ziegler-catalyzed copolymers were characterized by a faster crystallization rate and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single-site and Ziegler-catalyzed copolymers were explained based on their intermolecular comonomer distributions. Evaluating the effects of changes in the heating time or the oven temperature on the dimensional stability of molded parts was not as straightforward. The impact of these factors on the evolution of the temperature gradient across the melt as well as on the kinetics of crystallization needs to be evaluated and should account for structural changes in the material molecular structure due to degradation reactions as well as for the evolution of stiffness during the solidification process.
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Bibliography: p. 99-114
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Citation
Medina, A. (2007). Dimensional stability of single-site and ziegler-natta ethylene copolymers in rotational molding (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/1726