Browse Articles

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

Polyacrylonitryle (PAN) fibers are used on a large scale in textile industry but, some sorts of PAN fibers are valuable precursors for specific carbon fiber (CF) due to their best fitness to the purpose e.g. for high strength CF or for active CF. The application of PAN as precursors of CF is based on its specific structure and texture. Thus, the properties of PAN base CF strongly depend on PAN structure, texture and fiber morphology. In this respect, the paper addresses the modeling of PAN fiber structure and texture and, subsequently, estimation of structure and texture parameters based mainly on Wide Angle X-ray Diffraction (WAXD) technique but, on SEM and other data. The PAN fiber structure and texture parameters were calculated on the basis of classical X-ray diffraction theory applied to paracrystals. The preferred orientation parameters were calculated on a wide accepted model. The main structural parameter of PAN paracrystal d200, d201, and L200 and preferred orientation/texture parameters Z, [cos2 u], P (cos2 u) of the PAN fibers were determinated. Based on WAXD data it was calculated the PAN fiber crystallinity. As it results from the paper the WAXD technique seems to be the best from the efficiency-cost point of view for PAN structure and preferred orientation characterization. Keywords: WAXD, SEM, structure, preffered orientation, polyacrylonitryle fibers

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

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

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

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

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

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

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

The paper presents the main aspects of dynamic modelling of steel/plastic geared systems with high gear ratios in relation to the nonlinear parameters, such as time-varying mesh stiffness and load sharing alternation. Firstly, the specific aspects of geometrical design criteria for helical gear with small number of pinion teeth are presented in order to prevent the negative phenomena of the involute tooth generation and gear engagement. Then, the dynamic analysis is conducted to investigate the shared dynamic loads and dynamic transmission error of steel/plastic gear pairs along the line of action. The effects of the time-varying mesh stiffness are incorporated in the nonlinear dynamic model for numerical analysis. An improved model of the mesh stiffness is developed by including specific meshing characteristics of helical gear pairs. The variation of the dynamic transmission error is simulated and examined in relation to the teeth number, addendum modification coefficients and input torques. A computational algorithm is developed for instantaneous dynamic contact analysis. Keywords: plastic gears, high gear ratio, mesh stiffness, dynamic transmission error