Volume 48, Issue 2

Abstract: Radiation-induced polymerization can produce a wide variety of water-soluble polyelectrolytes which are used to increase the performance of the waste water treatment. The main goal of this study was to investigate the influence of the radiation dose (D), initiator concentration [I], sodium chloride concentration, total monomer concentration [TMC] and monomer ratio (R) on the physical and chemical properties of the acrylamide – acrylic acid based polyelectrolites, obtained by copolymerization in accelerated electron beam fields. In accordance with the established purposes, the polyelectrolyte must have high conversion coefficient (CC > 95%), low residual monomer content (Mr < 0.01), high intrinsic viscosity (hintr > 7 dL/g), low linearity coefficient (kH < 0.4) and a very good solubility in water. The experiments were performed using a linear accelerator facility (ALIN-10, built in NILPRP Accelerators Laboratory, Bucharest) of 6.23 MeV at irradiation doses between 0.9 kGy and 1.8 kGy at room temperature and ambient pressure. The best results were obtained at 1.5 kGy. Keywords: acrylamide, acrylic acid, ionizing radiation, polyelectrolytes

Abstract: Generally the fillers are used in order to diminish the amount of matrix for a certain composite structure but in the last period fillers are used to change also the basic properties of the material used as matrix, especially in the case of PMC (polymer matrix composites). The actual study is carried out in order to determine the influence of starch over the properties of epoxy resin and is based on the well known fact that starch is creating its own network inside of a matrix. In this case it is possible to talk about a nano-reinforcement of the matrix which might be used to form fiber reinforced materials with better mechanical or thermal properties. Due to its properties starch might be used also to ensure the dispersion of other fillers which generally tend to aggregate as CNT or ferrite and in this case it is necessary to know the upper limit of starch concentration for which the polymer properties are not changed or suffer negligible modifications. Not at least the presence of starch into the epoxy matrix might determine a higher alterability of the polymer making it easier to neutralize. Being easy to functionalize starch might be used as vector to transport positive and/or negative ions inside the polymer matrix via complex combinations which might lead to controllable electro-magnetic properties of the formed material or to multi-functional materials. The thermal, mechanical, tribological and electrical properties of starch/epoxy composites were studied using appropriate methods for materials with various starch concentrations. Key words: starch, epoxy, physical properties, mechanical properties

Abstract: In this paper, we determined a state of stress, which verifies the Cauchy equations of equilibrium, the conditions of continuity on the surfaces between layers and boundary conditions for a sandwich beam subjected to tensile test. We customized the relations, previously obtained, for a composite beam consisting of two layers. Using a mediation formula for strain and stress, we obtained a new formula for the calculus of the longitudinal modulus of elasticity, when the constituent materials have different Poisson ratios. Considering the maximum stress reached in the layers, we determined a formula, which helps us to determine the order in which the two layers will break. We did experimental measurements for test samples made from polyesteric resin, reinforced with fiber glass fabric, carbon and carbon-kevlar. Keywords: composite materials, characteristic curve, elasticity modulus

Abstract: This paper presents the influence of the position of an applied static load on a honeycomb sandwich with core made of resin impregnated honeycomb and caps made of epoxy resin. The nonlinear analyses were concentrated underneath the area of applied force and on the side of one wall of the honeycomb. The results obtained can be used to determine the effect of a small area impact on a honeycomb sandwich or to establish the stress distribution pattern during compression tests on the walls of a cellular structure. Keywords: honeycomb, sandwich, nonlinear, resin impregnated paper, epoxy resin

Abstract: This paper presents a study of the electrical conductivity of wood-polymer composites, determined by measurements done in DC and AC current. It is also noticed that the values of sdc, s’ and s” are strongly influenced by the nature of matrix and filler content of the composite. Finally, it is shown that samples of PE and PP matrices can be used for applications in power frequency electric fields (0 ... 100 Hz) and at temperatures between 30 and 90 °C. Keywords: compozite lemn-polimer, conductivitate electrica

Abstract: This work brings to the forefront local nonlinear problems that appear in junctions of beam structures made of plastics, subjected to heavy loads. We also revealed the notions of local problems and heavy loads. These theoretical aspects are explained with the help of two beam structures of various complexity, made of plastic, with local problem. The results are presented as stress maps, maps of mechanical strains, charts with the evolution of the states reached in the structure with the loading steps. We mentioned the situations that require local study; nonlinear problems classification is presented and the methodology for nonlinear calculus is described. Keywords: local problem, beam structure, heavy loads, nonlinearity

Abstract: The plastics components of a random sample of waste electrical and electronic equipment (WEEE) from small household appliances were identified and sorted. Moreover, by taking into account the practical needs for applications demanding medium to less severe restrictions from the standpoint of end-use characteristics, the mechanical and thermal properties are investigated for the four most used plastics in the EU, including acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile (SAN), polystyrene (PS) and polypropylene (PP), as well as for a mixture resulting on the basis of corresponding EEE market evaluations. The obtained results show that in the case of these materials, separated prior to processing, even if a minor decrease arises from the standpoint of thermal properties due to inherent degradation induced by processing, the problem is the decline of different mechanical properties. In fact, this appears to be the net result of different morphologies of components – the essentially amorphous-like nature of ABS, SAN and PS, and the semi-crystalline component (PP). Accordingly, the loss in mechanical properties was lower for the elastic modulus, higher in case of impact strength and intermediate for the tensile strength, when the components and the resulting blend properties are compared. Keywords: WEEE, plastics, recycling, thermal properties, mechanical properties

Abstract: The main characteristics of pressure sensitive adhesives are: peel, shear and tensile strength. This paperwork presents the shear tests and related phenomena of pressure sensitive adhesives based on polychloroprene rubber using both metallic and polymeric substrates. The influence of the main parameters which control the shear strength of the adhesive assemblies was studied: nature of the substrates, contact time and pressure. Keywords: pressure sensitive adhesives, elastomers, polychloroprene rubber

Abstract: The aim of this study was to test the hypothesis that microwave post-polymerization treatment influences the rise of hardness, flexural strength, impact strength and fracture toughness of autopolimerizing poly(methyl-methacrylate) by the decrease in residual monomer content. Specimens produced from a commercial acrylic denture reline resin were polymerized according to manufacturers instructions. Control group was left untreated, while other five were post irradiated in a microwave oven with different power and time settings, keeping the irradiation energy constant. Each group of samples was tested for mentioned mechanical properties. Enthalpies of polymerization were tested by differential scanning calorimetry (DSC), while the amount of residual monomer was determined using fourier transform infra-red spectroscopy (FTIR). Microwave post-irradiation resulted in increase of all tested mechanical properties. The highest mecanical properties were obtained with maximum irradiation power, which was confirmed by the results of ANOVA statistical analysis. It has been found that residual monomer content strongly influences the benefits in all tested mechanical properties, increasing biocompatibility. These results have been confirmed by the results of Scanning electron microscope (SEM) fracture surface analisys. Keywords: PMMA biopolymer, microwave post – irradiation, mechanical properties, residual monomer

Abstract: In this paper we present a new multiscale method for coupling molecular dynamics simulations (MD) with Monte Carlo (MC) simulations. The method is relevant for heat transfer phenomena at the molecular level with applicability in many domains such as polymer processing, crystallization process in moving polymer melt, complex flows near the interfaces, e.g. wetting, colloids near surfaces, drop formation, etc. First the theoretical background is described and after that simulation results between multiscale MD-MC and pure MD simulation are presented. At the end, comparisons between accuracy and computational costs of MD, MC and multiscale MD-MC simulations are outlined. Keywords: heat transfer techniques, polymers, Monte Carlo simulations, molecular dynamics,multiscale modeling

Abstract: Two types of cellulose fibers were obtained from microcrystalline cellulose (MCC) by acid hydrolysis and ultrasound treatment using different concentrations of sulphuric acid. The effect of acid concentration on cellulose fibers characteristics was studied by scanning electronic microscopy (SEM), dynamic light scattering (DLS), and X-ray diffraction analysis (XRD), pointing out interesting features from the standpoint of different end-use properties. An important increase of MCC crystallinity, from 51.4% to 74.3%, was observed by XRD analysis after the treatment with 60% sulphuric acid. The cellulose fibers were used as reinforcements in a polyvinyl alcohol (PVA) matrix and the crystallinity, transparency and mechanical properties of the resulted composites being investigated. The incorporation of the two types of cellulose fibers into PVA led to transparent composites films with improved mechanical properties. A steady improvement of the tensile modulus of PVA composites was observed with the increasing amount of fibers, an increase of 83.3 % being obtained with cellulose fibers resulted from MCC treatment with higher acid concentration (60%). Key words: PVA composites, cellulose nanofibers, acid hydrolysis of cellulose, mechanical properties

Abstract: This paper studies the influence of positioning angles of neoprene vibration isolation elements from the first elastic stage, upon the natural frequencies of the vibrator-roller type CVA-10. A simplified computing dynamic model with four degrees of freedom has been used in order to determine the influence of positioning angle on the natural frequency by means of experimental results. Keywords: neoprene elements, positioning angle, vibrator-roller, natural frequencies

Abstract: Dimethacrylate-based resins cured using a photoinitiator system are widely used as matrices in dental restorative composites. In this work, the thermal degradation of copolymers based on urethane dimethacrylate (UDMA) and bis-phenol A glycidyl dimethacrylate (BisGMA) were investigated by thermogravimetric analysis. Different weight ratios (100/0, 70/30, 50/50, 30/70 and 0/100) between the UDMA and BisGMA comonomers were employed. The camphorquinone – ethyl-4-dimethylaminobenzoate photoinitiator system was used. The degradation of copolymers occurs in three steps, contrarily to two steps for UDMA homopolymer and a single step for BisGMA homopolymer. Activation energies of the degradation processes were calculated using differential and integral isoconversional methods. The influence of copolymers composition on the activation energy as a function of the degradation conversion degree was studied. Keywords: thermal degradation, dimethacrylate copolymers, UDMA/BisGMA

Abstract: Aiming to develop a self-contained presentation regarding the thermorheodynamical behaviour of solid-like polymer materials, the Zener - Arrhenius model is used, the frequency or/and temperature dependences being considered in the case of stress-controlled conditions. Accordingly, the full set of the explicit general analytical relations for the primary characteristic thermorheodynamical quantities - the storage, loss and absolute compliances, as well as the loss factor - and the corresponding secondary ones - the storage, loss and absolute moduli - is obtained by using the appropriate rheological parameters including the low and high frequency limit values of the storage compliance, as well as the retardation time. These underlying analytical relations are provided in forms directly suitable for numerical simulation of frequency dependence in isothermal frequency - dependence, and as well for the temperature - dependence in isochronal circumstances. Keywords: analytical relations, characteristic thermorheodynamical quantities, stress-controlled conditions, Zener - Arrhenius model, isothermal and isochronal circumstances

Abstract: Bacterial cellulose offers an interesting potential as natural biodegradable alternative to glass and carbon fibers that are currently used in structural composite. In the present paper, bacterial cellulose produced by Gluconacetobacter Xylinus ATCC 23768 was used as reinforcement fiber to develop four new type composites. The composites were prepared using bacterial cellulose sheets and epoxy resin infused at 10, 15, 250 MPa. These materials were composed of 8 layers of BC pressed at 10 MPa, 8 layers of BC pressed at 15 MPa, 8 layers of BC pressed at 250 MPa and 28 layers of BC pressed at 10 MPa. The first three obtained materials were subject to uniaxial tensile tests, while the fourth material was subject to three-point flexural tests. By using this unique natural material and the resin infusion method it was possible to obtain a composite with the highest reported strength for bacterial cellulose based composites until now. The tensile strength revealed is of about 465 MPa and Young’s modulus of about 60 GPa. Keywords: bacterial cellulose, Acetobacter xylinum,Gluconacetobacter Xylinus Epoxy composite

Abstract: A series of thermotropic liquid crystalline phosphorus-containing copolyesters was prepared by polycondensation reaction starting from various ratios of 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)-1,4-benzene diol, 1, terephthaloyl bis-(4-oxybenzoyl-chloride), 2, and, 1,12-dodecanediol, 3. The molar ratio of aromatic bisphenol to aliphatic diol was varied in order to generate copolyesters with tailored physico-chemical properties. The chemical structure of the synthesized polymers has been confirmed by Fourier transform infrared and 1H NMR spectroscopy. The phase transitions of the phosphorus-containing copolyesters have been investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction measurements, structure–properties relationships being established and discussed. Keywords: phosphaphenanthrene; phase transitions; structure-property relations