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Oseli A.,Institute for Sustainable Innovative Technologies | Emri I.,University of Ljubljana
Key Engineering Materials | Year: 2014

This paper describes a novel apparatus for measuring dynamic bulk compliance B* (ω) of time-dependant materials. System can measure dynamic bulk compliance at room temperature, at pressures up to 100 ± 1,5 bar and frequencies from 100 Hz to 1000 Hz. Functionality of the apparatus is demonstrated by performing measurements of dynamic bulk compliance for two different materials, i.e., polyvinyl acetate (PVAc) and thermoplastic polyurethane (TPU). Measurements were conducted at room temperature, atmospheric pressure and frequencies from 100 Hz to 1000 Hz. Copyright © 2014 Trans Tech Publications Ltd, Switzerland. Source

Gergesova M.,University of Ljubljana | Saprunov I.,United Research and Development Center | Emri I.,University of Ljubljana | Emri I.,Institute for Sustainable Innovative Technologies
Rheologica Acta | Year: 2016

The closed-form shifting (CFS) algorithm is a simple mathematical methodology which determines the unique solution in the process of constructing master curves at selected reference temperature and pressure conditions. In a previous paper, the CFS algorithm has been fully described for monotonically increasing or monotonically decreasing functions only. This paper presents detailed steps of the generalized CFS methodology for non-monotonic functions, like the loss tangent. Performing shifting on the loss tangent, which does not require vertical shifting, is particularly important for materials which require vertical adjustment of dynamic viscoelastic functions, i.e., loss and storage moduli. Thus, based on horizontal shifting of the loss tangent, the CFS-based procedure of consecutive horizontal-vertical superposition for the storage modulus is proposed and analyzed. The analysis is done on the example of two synthetically generated non-monotonic tan delta segments and corresponding storage modulus segments in respect to different experimental parameters. It has been shown that the error brought by the shifting method into non-monotonic loss tangent and storage modulus master curves is twice smaller than the corresponding experimental noise level. © 2016 Springer-Verlag Berlin Heidelberg Source

Gonzalez-Gutierrez J.,University of Ljubljana | Oblak P.,University of Ljubljana | Von Bernstorff B.S.,BASF | Emri I.,University of Ljubljana | Emri I.,Institute for Sustainable Innovative Technologies
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2014

Polyoxymethylene (POM) is considered a high performance engineering polymer with many applications primarily in the automotive industry. Currently, POM has also found uses in powder injection molding (PIM) technology, where it acts as a carrier medium for metal or ceramic powders during the injection molding process, it is later removed and a metallic or ceramic piece is obtained after sintering. The main advantage of using POM in PIM technology is the faster debinding process compare to polyolefin-based feedstock, since POM sublimates into its monomer directly when exposed to an acid vapor. During the process of PIM, the binder has two contradictory requirements: viscosity should be as low as possible when in the molten state, but mechanical properties in the solid state, like toughness, should be as high as possible. One way to lower the viscosity is to use POM with lower molecular weights. In this work it has been observed that the viscosity follows a power law function as with other linear polymers, while the fracture toughness follows an exponential function of the average molecular weight. Therefore, a molecular weight can be chosen in a way that a compromise between low enough viscosity and sufficient fracture toughness can be reached. © The Society for Experimental Mechanics, Inc. 2014. Source

Oblak P.,University of Ljubljana | Gonzalez-Gutierrez J.,University of Ljubljana | Zupancic B.,University of Ljubljana | Aulova A.,University of Ljubljana | And 2 more authors.
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2016

In the plastics industry it has been common practice to mechanically recycle waste material arising from a production. However, mechanical recycling affect material mechanical properties and consequently quality of the end products; therefore it needs to be quantified. Mechanical recycling of high density polyethylene (HDPE) was simulated by one-hundred consecutive extrusions. After every cycle, portion of material was removed for the purpose of characterization. Solid mechanical properties of the material were characterized in terms of hardness and modulus measured with nanoindentation. Furthermore, shear creep compliance was measured to characterize the materials’ time-dependent mechanical properties in solid state. In addition, differential scanning calorimetry (DSC) measurements were performed in order to study structural changes through the degree of crystallinity. The results on hardness and modulus show deterioration of the material mechanical properties through the process of repeating recycling. This becomes increasingly evident after 10th extrusion. Similarly, shear creep compliance measurements show an unfavourable effect of mechanical recycling on the time-dependent mechanical properties. In this case, evident changes are visible in particularly after the 30th extrusion. After 100th recycling material mechanical properties reduces for about 20%. All those changes are well supported by changes of the degree of crystallinity. © The Society for Experimental Mechanics, Inc. 2016. Source

Gonzalez-Gutierrez J.,University of Ljubljana | Oblak P.,University of Ljubljana | Megen Z.M.,University of Ljubljana | Emri I.,University of Ljubljana | Emri I.,Institute for Sustainable Innovative Technologies
Polimery/Polymers | Year: 2015

The effect of average molecularweight (Mw) of polyoxymethylene (POM) on melt viscosity and solid state creep compliance were investigated. Viscosity follows the power function of Mw. Creep compliance results indicate that time-Temperature superposition applies to POM copolymers. Creep compliance in a short time (0.25 s) is independent of Mw, but in a longer time (10 years) it follows an inverse power law relation withMw, up to a critical value ofMw = 81 100, where creep compliance becomes independent of Mw. At intermediate time (17 min), similar to short one, no effect on susceptibility to creep compliance was observed. It was also stated that the activation energy is independent of Mw. Source

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