The paper presents the effect of polymer solution composition on the morphology, mechanical properties and drug permeability of the asymmetric polyamide 6 (PA6) membranes prepared by immersion precipitation. The effect of polymer solution composition on morphology, mechanical properties and permeability of the produced membrane is considered, since these properties are of relevance for drug delivery applications. PA6-formic acid-deionized water solutions were used to prepare membranes for further characterization with differential scanning calorimetry and scanning electron microscopy for morphology analysis, tensile testing and drug permeability tests. The results show that the amount of PA6 does not significantly affect morphology of the membrane, while having pronounced effect on tensile elastic modulus (50% increase). On the other hand, the concentration of formic acid in solution (dissolution intensity) influences crystallization dynamics and significantly changes the morphology of membrane (in the range of approximately 75-100 wt% of formic acid concentrations), consequently having effect on drug permeability. Asymmetric membranes are widely used in filtration and drug delivery application; however their morphology is rarely connected to the permeabilty and mechanical properties. This publication shows the interrelation between the production process parameters and the porous structure of the system, which consequently affects other properties. Thus, obtained results allow optimization of properties of the assymetric membrane by balancing between desirable mechanical properties and permeability.
COBISS.SI-ID: 16094747
Health monitoring systems for plastic based structures require the capability of real time tracking of changes in response to the time-dependent behavior of polymer based structures. The paper proposes artificial neural networks as a tool of solving inverse problem appearing within time-dependent material characterization, since the conventional methods are computationally demanding and cannot operate in the real time mode. Abilities of a Multilayer Perceptron (MLP) and a Radial Basis Function Neural Network (RBFN) to solve ill-posed inverse problems on an example of determination of a time-dependent relaxation modulus curve segment from constant strain rate tensile test data are investigated. The required modeling data composed of strain rate, tensile and related relaxation modulus were generated using existing closed-form solution. Several neural networks topologies were tested with respect to the structure of input data, and their performance was compared to an exponential fitting technique. Selected optimal topologies of MLP and RBFN were tested for generalization and robustness on noisy data; performance of all the modeling methods with respect to the number of data points in the input vector was analyzed as well. It was shown that MLP and RBFN are capable of solving inverse problems related to the determination of a time dependent relaxation modulus curve segment. Particular topologies demonstrate good generalization and robustness capabilities, where the topology of RBFN with data provided in parallel proved to be superior compared to other methods.
COBISS.SI-ID: 15013147
This paper presents the analysis of pressure dependence of three thermoplastic polyurethane (TPU) materials on vibration isolation. The three TPU Elastollan materials are 1190A, 1175A, and 1195D. The aim of this investigation was to analyze how much the performance of isolation can be enhanced using patented Dissipative bulk and granular systems technology . The technology uses granular polymeric materials to enhance materials properties (without changing its chemical or molecular composition) by exposing them to "self-pressurization," which shifts material energy absorption maxima toward lower frequencies, to match the excitation frequency of dynamic loading to which a mechanical system is exposed. Relaxation experiments on materials were performed at different iso- baric and isothermal states to construct mastercurves, the time-temperature-pressure interrelation was modeled using the Fillers-Moonan-Tschoegl model. Dynamic material functions, related to isolation stiffness and energy absorption, were determined with the Schwarzl approximation. An increase in stiffness and energy absorption at selected hydrostatic pressure, compared to its stiffness and energy absorption at ambient conditions, is represented with K_k(p, [omega]), defining the increase in stiffness and K_d (p, [omega]), defining the increase in energy absorption. The study showed that close to the glassy state, moduli of 1190A and 1195D are about 6-9 times higher compared to 1175A, whereas their properties at...
COBISS.SI-ID: 15825947
The viscosity of feedstock materials is directly related to its processability during injection molding; therefore, being able to predict the viscosity of feedstock materials based on the individual properties of their components can greatly facilitate the formulation of these materials to tailor properties to improve their processability. Many empirical and semi-empirical models are available in the literature that can be used to predict the viscosity of polymeric blends and concentrated suspensions as a function of their formulation; these models can partly be used also for metal injection molding binders and feedstock materials. Among all available models, we made a narrow selection and used only simple models that do not require knowledge of molecular weight or density and have parameters with physical background. In this paper, we investigated the applicability of several of these models for two types of feedstock materials each one with different binder composition and powder loading. For each material, an optimal model was found, but each model was different; therefore, there is not a universal model that fits both materials investigated, which puts under question the underlying physical meaning of these models.
COBISS.SI-ID: 14692379
Flow of granular materials is a complex process but it is important to measure, because the flow of granular material during processing, handling and transportation strongly influences the quality of the final product and its cost. Flowability of granular materials depends on the characteristics of the material and on the conditions at which flow is occurring. Existing methods of measuring flowability of powders are described in this paper, and a new methodology is introduced to measure friction between granular materials under pressure induced with uniaxial compression. Apparatus also allows analysis of conditions at which granular material starts to flow when exposed to uniaxial compressive load, i.e., zero-rate flowability. We call the apparatus the Granular Friction Analyzer (GFA). The concept of the GFA was tested by measuring four different materials with different average particle sizes. It was observed that as the particle size decreases so does its zero-rate flowability. This is in agreement with powder literature. Therefore, it can be concluded that in general the GFA method can be a very useful tool to study friction between granular materials and conditions at which the granular material flow initiates, i.e. zero-rate flowability of powders under pressure. However, further improvements are required to increase its sensitivity and accuracy.
COBISS.SI-ID: 14351899
In plastics industry it is a common practice to mechanically recycle waste material arising from production. However, while plastics are mechanically recycled, their mechanical properties change. These changes may affect material processing conditions and quality of the end products; therefore they need to be quantified. In this study, mechanical recycling of high density polyethylene (HDPE) was simulated by one-hundred (100) consecutive extrusions cycles. During extrusion, processability of virgin HDPE and its recyclates was studied by recording the processing conditions, i.e. melt pressure and extrusion torque, while after preparation of the recyclates, melt flow index measurements (MFI), small amplitude oscillatory shear tests (rheological properties), and differential scanning calorimetry measurements (DSC) of thermal properties were performed. Also, mechanical properties in solid state were characterized in terms of hardness and modulus measured by nanoindentation, and finally, shear creep compliance was measured to characterize the materials' time-dependent mechanical properties and its durability in solid state. In addition, gel permeation chromatography (GPC) and solubility tests were implemented to study changes in the material structure. The results on rheological and MFI measurements indicate significant structural changes in the material that occurred during the first 30 extrusion cycles. Those changes affect material processability which is as well supported by the recorded processing pressure and torque. On the other hand, processing did not significantly affect material thermal properties. Results on hardness and modulus show deterioration of the material mechanical properties after 10th reprocessing cycle. Similarly, shear creep compliance measurements showed an unfavourable effect of mechanical recycling on the time-dependent mechanical properties, particularly after the 30th extrusion cycle. In addition, results suggested chain branching as a dominating mechanism through first 30 extrusion cycles, domination of chain scission afterwards and also presence of cross-linking after 60th extrusion cycle.
COBISS.SI-ID: 13935131
To predict durability of polymeric structures an information on polymers long-term properties in the form of relaxation modulus and/or creep compliance is required. It iswell known that determination of relaxation or creep properties from experimental data isan inverse problem, which, due to presence of experimental errors in input data, becomesill-posed. To find a stable solution using standard integration schemes is practically impos-sible. In this paper we propose a hands-on methodology which bypasses the solution ofill-posed integral equation and allows finding long-term relaxation or creep properties fromsimple constant strain rate or constant stress-rate experiments performed at different temper-atures. The proposed approach can be applied not only for characterization of viscoelasticmaterials in solid state but can also be used for prediction of time-dependent properties ofpolymer melts. The paper presents the detailed steps of the proposed method as well as itsvalidation on several simulated and real experimental data. It has been shown that the pro-posed approach can accurately reconstruct the desired long-term time-dependent propertiesobtained in traditional way (i.e., from step loading).
COBISS.SI-ID: 13309211
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 temperaturesand/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 methodology (CFS)which completely removes ambiguity related to the manual shifting procedures. This paper presents the derivation of the shifting algorithm and its validation using several simulated- and real- experimental data. It has been shown that error caused bz shifting performed with CFS is at least 10-50 times smaller then the underlying experimental error.
COBISS.SI-ID: 11702043
The effects of various drying techniques, such as air, oven, freeze, and spray drying, on the morphological, thermal, and structural behaviors of two different nanofibrillated cellulose (NFC) materials were investigated. Field emission scanning electron microscopy (FE-SEM) observations indicated an interlaced network formation of predominantly in-plane fibrillar orientation for air- and oven-dried samples, while freeze and spray drying resulted in the formation of coarse and fine powder fractions. Comparison of redispersed powders obtained by freeze and spray drying indicated that aggregation phenomena are significantly reduced in freeze-dried specimens. Rheological and sedimentation analysis revealed that the freeze-dried NFC powders are more stable than spray-dried NFC powders when redispersed in water. Aggressive dehydration processes, such as freezing or heating, significantly influence the thermal stability of the dried cellulose samples. On the contrary, the crystallinity properties of dried NFC materials are very similar regardless of the drying treatment
COBISS.SI-ID: 2176905
The use of polymers as engineering materials has significantly increased in the past decades. Their increased usage is the result of better engineering, economic and environmental properties that can complement or substitute conventional materials, such as metals. However, during the production and utilization phase, polymers are exposed to different environmental conditions such as temperature, pressure, humidity, etc., that profoundly affect their behavior. Therefore, the main idea behind this chapter, “Effect of temperature on mechanical properties of polymers,” is to obtain comprehensive insight on behavior of polymers at different environmental conditions, with emphasis on temperature, which is important in numerous engineering processes (e.g., injection molding, extrusion, blow molding, etc.) and applications (e.g., stress–strain analysis, predictions of durability of products, etc.). Considering environmental effects is crucial for exploiting full potential of polymeric materials, their safe usage, and to extend their durability. In this respect, the chapter addresses several coherent topics related to the effect of temperature on mechanical properties of polymers. The starting point is the molecular structure of polymers (i.e., bundles of molecules), which significantly differs from metallic structure (i.e., crystalline structure), and is the main culprit for their “unconventional” behavior in the sense of their mechanical properties and profound sensitivity to the environmental conditions. Following the molecular structure is the theory of viscoelasticity where the theoretical and mathematical background needed to determine mechanical properties of polymers is explained. This part also contains interrelations between different material functions that are result of different types and modes of loading. Comprehending both topics is of key importance for the last part related to the effect of temperature on behavior of polymers. This part contains description of molecular mechanisms that govern the processes on macro scale (e.g., engineering scale), observed through he changes in volume and mechanical properties. The publication in the section of Planar Problems within Springer Encyclopedia of continuum mechanics aims at delivering knowledge on polymeric materials to the wide public. The chapter covering effects of temperature is accompanied by the chapter elaborating effects of pressure on material properties of polymers [COBISS.SI-ID 16177179].
COBISS.SI-ID: 16177435