Oskar Bera

Oscillatory shear rheology of polystyrene melts filled with carbon black and fullerene

Abstract Fullerene C60 and submicrometre sized particles of carbon black were used to investigate the size effect of filler particles on the rheological behaviour of polydisperse polystyrene melts. The addition of 5 wt-% carbon black increased the viscosity of all the composite melts relative to their polymer matrixes. The effect of 5 wt-% fullerene nanoparticles on the viscosity of composites depended on the molecular weight distribution of the polymer matrixes. In the polymer with a high fraction of polymer chains below the critical molecular weight for entanglement, the fullerene addition increased the viscosity, similar to carbon black addition. As the molecular weight distribution of the polymer matrix moved to higher values, the influence of fullerene addition on melt viscosity changed, and the viscosity of the nanocomposite was similar to that of the polymer matrix. The effect of fullerene addition on the melt viscosity of the coarse particle composite was also investigated.

The influence of silica nanoparticles on thermal degradation and mechanical properties of nanocomposites based on aliphatic polyurethanes

Nanocomposites based on aliphatic polyurethanes have recently attracted a lot of attention regarding economical and ecological aspects, due to their improved thermal and mechanical properties. The aim of this paper was to investigate the influence of silica nanoparticles, differing in size and specific surface, on thermal stability and degradation, lifetime and mechanical characteristics of the obtained nanocomposites. Two series of nanocomposites based on aliphatic polyurethanes were obtained by using a single-step procedure and by addition of silica nanoparticles of types A380 or N999 at different loadings (0,0 0.15, 0.5, 1,0 and 3.5 wt.%). It was found that the increase in heating rate caused shifting of the onset temperature to higher values (from 283 to 312 °C). According to the shape of DTG curves, it was observed that the degradation mechanism of prepared nanocomposites consists of two overlapping processes, related to the scission of hard and soft building blocks. Based on DTG results, the addition of larger N999 silica nanoparticles induced lower thermal degradation, shifting the maximum rate temperatures of the first and second degradation stages to lower values, and caused the change in the degradation mechanism. Addition of smaller silica nanoparticles (A380) did not significantly affect the mechanism of the degradation reaction, indicating homogeneity of the obtained nanocomposites. The presence of A380 nanoparticles improved thermal stability of nanocomposites, by increasing the onset temperature from 286 °C for the pristine elastomer to 303 °C for the sample containing 3.5 wt.% of silica. Existence of interactions of A380 silica nanoparticles with hard and soft phases was observed, based on the increase in the maximum rates of the first and second degradation steps. The activation energy of thermal degradation of polyurethanes modified with A380 silica nanoparticles was obtained by using the Flyn-Wall and Toop models. Dependence of the activation energy and the lifetime of nanocomposites based on aliphatic polyurethanes on the silica content were estimated. The highest Ea values (determined for 1 and 5 % weight loss) were found for nanocomposites containing 0.5 and 0.15 wt. % of A380 silica nanoparticles (121 and 161.2 kJ/mol). A negative effect of the silica addition on mechanical properties of nanocomposites was observed. The polyurethanes containing smaller SiO2 particles (A380) had a higher tensile strength, elongation at break and hardness as compared to the elastomers filled with larger silica nanoparticles (N999). [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. III 45022]

MATERIAL AND ENERGY FLOW MANAGEMENT- LIFELONG LEARNING COURSES EXPERIENCES

Material and energy flow management is among the most important factors for the economic growth of a society in harmony with nature and it will play the main role only in the area of sustainable development. The three lifelong learning courses were developed in order to introduce this concept to the industry professionals and administration at the city and regional level. The main goal was to heighten awareness of participants to this topic, as in Serbia it is relatively new approach in corporate and administrative management. The results of the courses confirmed the interest in this kind of lifelong learning courses with suggestion to join it in a single course with emphasise on the specific participant’s problems.

Primena veštačkih neuronskih mreža za matematičko modelovanje uticaja sastava i uslova proizvodnje na svojstva PVC podnih obloga

The application of PVC floor coverings is strongly connected with their end-use properties, which depend on the composition and processing conditions. It is very difficult to estimate the proper influence of the production parameters on the characteristics of PVC floor coverings due to their complex composition and various preparation procedures. The effect of different processing variables (such as time of bowling, temperature of bowling and composition of PVC plastisol) on the mechanical properties of PVC floor coverings was investigated. The influence of different input parameters on the mechanical properties was successfully determined using an artificial neural network with an optimized number of hidden neurons. The Garson and Yoon models were applied to calculate and describe the variable contributions in the artificial neural networks. [Projekat Ministarstva nauke Republike Srbije, br. III 45022]

Modification of epoxy resins with thermoplastic segmented polycarbonate-based polyurethanes

In this work, epoxy hybrid materials were synthesized by addition of thermoplastic segmented aliphatic polyurethanes with good elastic properties. The modified epoxy samples were obtained by curing of previously homogenized mixture of prepared polyurethane melts, epoxy resin and crosslinking agent Jeffamine D-2000. The influence of different weight content of polyurethanes (5, 10 and 15 wt. % compared to pure epoxy resin) as well the influence of different hard segments of elastomers (20, 25 and 30 wt. %) on the curing of modified epoxy systems was studied. The curing was followed by differential scanning calorimetry (DSC), in dynamic regime from 30 to 300°C, at three heating rates (5, 10 and 20°C/min). With the increase of hard segments content of polyurethanes added in higher concentration (10 and 15 wt. %) into epoxy matrix, the temperature of maximum ratio of curing was shifted to lower values (from 205 to 179°C). Obtained DSC data were analyzed using two integral methods (Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose) and one differential kinetic model (Friedman). The significant differences were observed in the second part of the epoxy curing (for the reaction degrees higher than 60 %), where the values of activation energies remarkably increase. The addition of polyurethane elastomers retarded the curing process due to decreased mobility of reactant molecules caused by higher viscosity of reaction mixture. By detailed analysis of determined kinetic parameters, it is concluded that the influence of slow diffusion is more pronounced in the presence of thermoplastic polycarbonate-based polyurethanes, which confirmed their effect on the mechanism of epoxy curing. The highest tensile strength and hardness showed the DGEBA modified with the polyurethane with highest hard segment content. Increasing the hard segment content of polyurethane and its concentration in matrix, the tensile strength of modified epoxy was increased. The elongation at break of modified epoxy samples was significantly improved by addition of polycarbonate-based polyurethanes with low hard segment content, due to higher content of flexible soft segment chains. [Projekat Ministarstva nauke Republike Srbije, br. III 45022), i Pokrajinski Sekretarijat za nauku i tehnoloski razvoj (projekat 114-451-2396/2011-01). Autor iz Praga duguje zahvalnost „the Grant Agency of the Czech Republic“ (Czech Science Foundation, project No. P108/10/0195).]

Transparent Tetragonal Zirconia Ceramics by Colloidal Processing of Nanoparticle Suspension

Colloidal processing was applied to a commercial 5 vol% 3Y-ZrO2 nanosuspension with a particle size of 10-15 nm. The nanosuspension was concentrated by evaporation or by the newly developed method of osmotic dehydration. The viscosity and stability of concentrated suspensions were investigated. The concentrated nanosuspension prepared by osmotic dehydration was consolidated by centrifugation in non-porous moulds. The dried deposit had a relative density of 46% and pores ranged from 4 to 8 nm. This deposit was densified by pressureless presintering to closed porosity, followed by hot isostatic pressing in order to obtain transparent ceramics. After sintering, the tetragonal zirconia retained the nanocrystalline structure with an average grain size of 65 nm and an in-line transmission of 25 % (at 633 nm wavelength and 0.5 mm plate thickness).