B. Zupančič

The closed form t-T-P shifting (CFS) algorithm

Time-dependent material functions of engineering plastics within the exploitation range of temperatures extend over several decades of time. For this reason material characterization is carried out at different temperatures and/or pressures within a certain experimental window. Using the time-temperature and/or time-pressure superposition principle, these response function segments can be shifted along the logarithmic time-scale to obtain a master curve at selected reference conditions. This shifting is commonly performed manually (“by hand”) and requires some experience. Unfortunately, manual shifting is not based on a commonly agreed mathematical procedure which would, for a given set of experimental data, yield always exactly the same master curve, independent of person who executes the shifting process. Thus, starting from the same set of experimental data two different researchers could, and very likely will, construct two different master curves. In this paper, we propose a closed form mathematical ...

A Note on Automated Time-Temperature and Time-Pressure Shifting

Time-dependent material functions of engineering plastics within the exploitation range of temperatures extend over several decades of time. For this reason material characterization is carried out at different temperatures and/or pressures within a certain experimental window, which for practical reasons extends typically over four decades of time. For example, when relaxation experiments in shear, are performed at different constant temperatures and/or pressures, a set of segments is obtained. Using the time-temperature and/or time-pressure superposition principle, these segments can be shifted along the logarithmic time-scale to obtain a master curve at a selected reference conditions. This shifting is commonly performed manually (“by hand”), and requires some experience. Unfortunately, manual shifting is not based on a commonly agreed mathematical procedure which would, for a given set of experimental data, yield always exactly the same master curve, independently of a person who executes the shifting process. Thus, starting from the same set of experimental data two different researchers could, and very likely will, construct two different master curves. In this paper we propose mathematical methodology which completely removes ambiguity related to the manual shifting procedures. Paper presents the derivation of the shifting algorithm and its validation using several simulated- and real- experimental data.

The effect of processing conditions on the durability of polymer products

The work discusses new possibilities of modifying the functionality of polymer products by changing their material structure during processing and consequently the time-dependent properties which define the durability of the final product. The research addresses two questions: 1) does current industrial-scale polymer processing equipment allow processing in conditions which enhance the nonlinear processes during structure formation; 2) what is the magnitude of the structural changes and what, consequently, are the time-dependent mechanical and other physical properties of the final polymer product which can be achieved by changing processing conditions in the industrial environment? By analysing the low density polyethylene (LDPE) extrudates produced with a laboratory extruder PolyLab OS, which simulates real industrial processing conditions, we observed that changing the processing conditions in the industrial environment improves the durability (time-dependency) of extrudates by several orders of magnitude. This opens new possibilities in the field of modifying the functionality of polymer products, and hence, better competitiveness on the world market.

Mechanical Properties of Extensively Recycled High Density Polyethylene

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.

Thermo-Mechanical Investigation of Free Volume Theory for Polyamie-6

Polymers always show time-dependent mechanical properties. In order to use polymers in engineering applications, long-term mechanical propertes should be characterized. Free volume theroy is the mostly used theory to predict and model the mechanical properties of polymers. The effect of temperature is modelled thorugh William-Landel-Ferry (WLF) equation, whereas, the combined effect of temperature and pressure is modelled by Filler-Moonan-Tschoegl (FMT) equation. Both of the models are based on free volume theory. A set of expermentations were performed to investigate the validity of free volume concpet for one of the most important engineering polymer; i.e. Polyamide-6.

Sterilization effect on structure, thermal and time-dependent properties of polyamides

This article studies the effect of different sterilization techniques on structure, thermal and time-dependent mechanical properties of polyamide 6 materials. We used two different types of polyamide 6 material: conventional polyamide 6 with monomodal molecular mass distribution and modified with bimodal one. Samples were prepared and properly shaped in order to observe the mentioned features before and after sterilization with three different techniques, namely sterilization with autoclave, ethylene oxide, and hydrogen peroxide plasma. Optical microscopy, differential scanning calorimetry and torsional creep testing were used for the properties investigation. As a main criterion to evaluate the effect of sterilization durability of materials was used. There was observed significant difference between monomodal and bimodal PA6 materials regarding their morphology and consequently thermal and mechanical properties: bimodal material exhibits more complex structure than chemically identical monomodal one. The results of analysis did not show considerable influence on both materials structure and thermal properties. However, there was observed strong effect on creep behavior. The most significant effect is detected for monomodal material sterilized by ethylene oxide, while creep compliance of the bimodal material did not change much due to any of applied sterilization techniques. Thus, modification the complexity of the material inherent structure improves sensitivity to sterilization.

On the Behavior of Dynamically Loaded Polymeric Materials

This paper presents the analysis of time‐dependent behavior of polymeric material under cyclic loading, modeled with the tooth‐like loading conditions in terms of the shear stress that are determined by the operating frequency and the ratio between unloading and loading phase within one loading cycle. Assuming viscoelastic behavior of the material, the intensity of strain accumulation process is analyzed regarding the effects of the operating frequency, and number of loading cycles for series of time‐dependent material properties expressed with discrete retardation spectrum. Results show that by knowing spectrum distribution of the time‐dependent material property we may predict the critical operating frequencies that may lead to the highest level of accumulated strain.

The Modification of Time‐Dependent Mechanical Properties of Polyamides due to Sterilization

We examine the effect of sterilization on functionality and durability of Polyamide 6. Nowadays there are several applications of this material in medicine in a form of surgical sewing material, vascular catheters and other implants. Understanding the time‐dependent behavior of PA‐6 is critical in predicting the durability of different medical products made from this polymer. We show that two PA‐6 materials having different initial kinetics and processed with the same technology, when exposed to sterilization, change their time‐dependent mechanical properties, and hence the durability in significantly different ways.

The Strain Accumulation Process in Periodically Loaded Polymer Based Products due to Viscoelastic Behavior of Polymers

We are interested in strain accumulation process in polymeric materials when exposed to periodic (cyclic) loading. Within each loading cycle material undergoes a combination of the creep and relaxation process. At certain conditions the retardation process between two loading cycles cannot be fully completed. Consequently strain starts to accumulate, which leads to hardening of the material and ultimately to the failure of polymeric product. Taking into consideration the importance of the mechanical spectrum of polymeric material for predicting durability of dynamically loaded polymeric products, [1], we present the analysis of the effect of the “shape” of material time‐dependent properties, expressed with the mechanical spectrum, on the process of strain accumulation under periodically applied loading. We observe, that spectrum shape defines the intensity and the magnitude of strain accumulation process. For certain type of material the accumulated strain will almost certainly lead to the failure of the ...

TIME‐DEPENDENT MECHANICAL BEHAVIOR OF PA6 NANOCOMPOSITES WITH TITANATE NANORIBBONS

We investigate to which extent we may modify time‐dependent mechanical behavior and other physical properties of polymeric product in solid state with the addition of titanate nanoparticles into polymeric matrix by simple melt compounding procedure without the presence of any processing additive. The experimental analysis of nanocomposites, prepared from PA6 matrix and titanate nanoribbons by simple melt compounding procedure, shows that the addition of titanate nanoribbons into PA6 matrix significantly slows down the creep process of host matrix, and hence improves its time‐ and temperature‐stability. This effect rapidly diminishes with increasing particle concentration.