Jelena Pavličević

Poly(methyl-methacrylate) nanocomposites with low silica addition

STATEMENT OF PROBLEM Poly(methyl-methacrylate) (PMMA) represents the most popular current denture material. However, its major drawbacks are insufficient ductility and strength. PURPOSE The purpose of this study was to improve the mechanical properties of PMMA in denture base application by adding small quantities of nanosilica. MATERIAL AND METHOD Silica nanoparticles were added to the liquid component of the tested materials. The standard heat polymerizing procedure was followed to obtain 6 PMMA--silicon dioxide (/SiO2) concentrations (0.023%, 0.046%, 0.091%, 0.23%, 0.46%, and 0.91% by volume). Microhardness and fracture toughness of each set of specimens was compared with the unmodified specimens. Furthermore, differential scanning calorimetry and scanning electron microscopy analyses were conducted, and the results obtained were correlated with the results of mechanical properties. RESULTS It was found that the maximum microhardness and fracture toughness values of the materials tested were obtained for the lowest nanosilica content. A nanosilica content of 0.023% resulted in an almost unchanged glass transition temperature (Tg), whereas the maximum amount of nanosilica induced a considerable increase in Tg. A higher Tg indicated the possible existence of a thicker interfacial layer caused by the chain immobility due to the presence of the particles. However, scanning electron microscopy results demonstrated extensive agglomeration at 0.91% nanosilica, which may have prevented the formation of a homogenous reinforced field. At a nanosilica content of 0.023%, no agglomeration was observed, which probably influenced a more homogenous distribution of nanoparticles as well as uniform reinforcing fields. CONCLUSIONS Low nanoparticle content yields superior mechanical properties along with the lower cost of nanocomposite synthesis.

Preparation and Thermal Stability of Elastomers Based on Irregular Poly(urethane-isocyanurate) Networks

The objective of this study was to investigate the thermal stability of poly(urethane-isocyanurate) networks with increasing amount of dangling chains. In order to improve thermal stability of elastomeric materials, networks were prepared by adding some heat resistant isocyanurate rings as a junction points by catalytic cyclotrimerisation of telechelic diisocyanates. The thermal degradation kinetics of samples has been studied by means of high-resolution thermal analyzer SDT Q600 TA Instruments, under nitrogen atmosphere. The activation energy for the two step reactions of thermal decomposition of different species was calculated and compared.

The Phase Structure of Novel Polycarbonate-Based Polyurethane-Organoclay Nanocomposites

Polycarbonate-based polyurethane nanocomposites were prepared using one step procedure by addition of either organically modified bentonite or montmorillonite (1 wt. %). All aliphatic components (polycarbonate diol, hexamethylene-diisocyanate and 1,4-butane diol) were used as reactants. The hard segment content of obtained thermoplastic polyurethanes was 30 wt. %. Scanning electron microscopy (SEM) was performed to investigate the morphology of obtained hybrid materials. The structure of synthesized elastomers was studied by Fourier transform infrared spectroscopy (FTIR). In order to obtain the degree of phase separation and investigate the hydrogen bonding constitution, deconvolution of –NH and –C=O IR regions was done, using Gaussian equations. It was determined that the degree of phase separation is not influenced by addition of organoclays, indicating uniform dispersion of silicate layers in the polyurethanes, which was also confirmed by SEM experiments.

Thermal stability of segmented polyurethane elastomers reinforced by clay particles

The aim of this work was to determine the influence of clay nanoparticles on thermal properties of segmented polyurethanes based on hexamethylene- diisocyanate, aliphatic polycarbonate diol and 1,4-butanediol as chain extender. The organically modified particles of montmorillonite and bentonite were used as reinforcing fillers. The structure of elastomeric materials was varied either by diol type or chain extender content. The ratio of OH groups from diol and chain extender (R) was either 1 or 10. Thermal properties of prepared materials were determined using modulated differential scanning calorimetry (MDSC). Thermal stability of obtained elastomers has been studied by simultaneously thermogravimetry coupled with DSC. The glass transition temperature, Tg, of soft segments for all investigated samples was about -33°C. On the basis of DTG results, it was concluded that obtained materials were very stable up to 300°C.

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).]

The investigation reaction kinetic for polyurethanes based on different types of diisocyanate and castor oil

The formation of polyurethanes based on vegetable oils is very complex and thus for industrial production of this materials it is important to determine the optimal temperature for polymerisation and finally to obtain materials with the proper mechanical properties. The goal of this work was to assess the kinetic of catalysed and noncatalysed reactions for polyurethanes based on castor oil as the polyol component and different types of diisocyanates. Due to the presences of hydroxyl groups on ricinoleic acid, castor oil is suitable for polyurethane preparation. The differential scanning calorimetry has been employed to study the polyurethane formation reaction using Ozawa isoconversion method. It was estimated that the catalyst addition decreases the activation energy. The highest reduction of activation energy was observed for the reactive systems with hexamethylene diisocyanate. Validity of obtained kinetic model was examined by FTIR spectroscopy following the apsorption of reactive groups. Obtained results of mechanical characteristics of the polyuretahane networks (with different NCO/OH ratio) confirmed that applied method could be used for prediction of optimal reaction condition in polyurethane networks synthesis.