Volume 48, Issue 4

Abstract: The objective of this paper consists in the minimization of warpage problems which depend on the process parameters during injection molding of thin wall plastic parts using an optimization technique involving Taguchi method, S/N ratio and ANOVA method. For this purpose, a number of experiments are carried out by utilizing the combination of process parameters based on a three-level L9 Taguchi orthogonal array table. To achieve the minimum warpage, the optimum level of process parameters is determined. A drain with 8 sockets is investigated, where melt temperature, mold temperature, injection speed, and packing pressure are selected to be the process variables. The results show that the optimum parameters that can minimize the warpage problem are: melt temperature (190°C), mold temperature (30°C), injection speed (70 mm/s), and packing pressure (55 MPa). The most effective factor on the warpage is packing pressure with a general influence of 55.71%. Keywords: plastic injection molding; thin wall parts; warpage; Taguchi method; S/N ratio; ANOVA method; optimization

Abstract: The options to deposit chemically a functional composite copper-diamond coating from trilonic electrolyte have been studied with respect to textile polyethylene terephthalate substrate. The optimal basic composition of the electrolyte for surface treatment and the optimal conditions of deposition has been found from the viewpoint of the deposition rate, respectively the thickness of qualitative layer of the copper matrix. A systematic study of the influence of different types and concentrations of surface active agents (SAA) and different temperatures of the electrolyte at continuous air flow stiring has been realized, aimed co-deposition of copper metal-matrix composite, containing diamond particles (sizes, varying within the range 3-70mm). Keywords: coating; fabrics/textiles; surface treatments; metal-matrix composites (MMCs); functional composites

Abstract: In this article, some advantages and disadvantages of the incorporation method of azobenzene moieties in several polymer matrices (polymethacrylate, poly(styrene-alt-maleimide), poly(oxazoline-co-methacrylate), poly(methacrylate-co-methacryloxypropyl trimethoxy-silane)) are highlighted. Their physical properties (molecular weights, glass transition temperatures, chromophore load, solubility, and solvatochromism) are discussed with regard to their macromolecular architecture. Solvent effect on the UV-Vis absorption spectra of azo-polymers was investigated using Reichardt’s solvent polarity scale and Kamlet-Taft multiparameter equation. Kamlet-Taft equation revealed that hydrogen bond acceptor properties and dipolarity/polarizability of the solvent have a significant contribution upon the solvatochromic behaviour of azopolymers. Keywords: azobenzene, side-chain polymers, solvatochromism, Kamlet-Taft equation

Abstract: A series of six new experimental light-curing dental composites based on two Bis-GMA-type oligomers, quartz and nano-fluorohydroxyapatite (nano-FHAP), in different ratios, were prepared. The paper refers to the characterization of organic and inorganic phases from the uncured composite pastes and determination of the properties of the light-cured composite materials. Differential scanning calorimetry was used to measure the heat of polymerization of the monomer mixtures. Investigation of nano-FHAP was made by transmission electron microscopy TEMBF, HRTEM and X-ray diffraction. The mechanical strengths, water sorption and solubility were determined. We came to the conclusion that for the same monomer mixture, the water sorption and solubility values increased with the content of nano-FHAP in the composite material, but there remained in the limits of ISO 4049/2000 for the compositions comprising up to 50% nano-FHAP in the filler. The highest mechanical strengths were obtained in the case of composites containing 75% quartz in the filler. Keywords: dental nanocomposites, extent of polymerization, mechanical properties, water sorption, solubility

Abstract: Fibre Reinforced Polymers (FRP) confinements on square and rectangular sections behave differently from confined circular sections. Confining inward pressure over the entire circular section is uniform, whereas in non-circular sections confining pressure development is inconsistent. As a result, these sections could be divided into the distinct properly and partially confined areas. Presence of these partially confined regions would much reduce influence of FRP confinements to increase concrete’s peak strength and ultimate strain. To evaluate confinement effectiveness, effect of mentioned areas must be taken into account precisely. This paper describes a theoretical model adapted to seismic strengthening and retrofit of the square section RC columns confined with advanced composite materials. The model is the author’s earlier proposed one calibrated with cautious survey on the existing test data. This model takes into account geometrical parameters of the column shape such as overlapping length and corner rounding radius; furthermore both concrete and FRP confinement mechanical properties. Finally it proposes a new shape factor for the square sections which plays a major role in effectiveness evaluation of FRP confinement. Keywords: Fibre Reinforced Polymers, square section, Reinforced Concrete columns, strengthening

Abstract: Lignin, a natural renewable material, was incorporated in a series of polydimethylsiloxane/silica composites. Besides the effect as bulk filler, the presence of lignin determines the change of some properties among them being surface properties that were evaluated by contact angle and water sorption measurements. The contact angles were determined in static regime by using the sessile drop technique. Dynamic water vapour sorption was applied to register the sorption/desorption isotherms and kinetic curves. The sorption type and maximum vapour sorption capacity were evaluated based on these data. The results were correlated with the composition of the prepared materials. Keywords: surface properties; siloxanes; composites; lignin

Abstract: Organic biopolymers are a source of raw material for agriculture, whereas the composition of protein wastes provides sufficient elements to improve the composition and remediation of degraded soils and plants by exploiting elements, such as nitrogen, calcium, magnesium, sodium, potassium etc. From processing one tone of raw hide, 75% is discarded, of which 50% could be further used as organic fertilizer. Protein biopolymers were obtained by means of an innovative enzymatic procedure for waste processing. In combination with polyacrylamide, acrylic, maleic, cellulose, starch etc. are used in the agricultural field. We characterize the protein biopolymers by specific morpho-structural analysis methods (UV-Vis, IR spectroscopy, thermal analyses, X-ray diffraction, microscopy).The novelty of this work is the biochemical combination process of synthetic polymers with organic biopolymers from tanneries with viable and interesting applications in agriculture. Keywords: biopolymer, protein wastes, tannery, soil, structural analysis

Abstract: Functionalized dental implants represent an advanced approaching in implantology, aiming to improve the biointegration and the long-term success of surgical procedures. In this article are reported the synthesis of hydroxyapatite (HA) and Poly(methyl methacrylate) (PMMA) thin films on metallic substrates – used as dental implant-type structures – by pulsed laser deposition (PLD) method. Plasma and plume evolution of ablated HA and PMMA species was registered through optical analyze by fast CCD camera. Numerical simulation performed using a simple hydrodynamic model properly reproduce this type of plasma behaviour. SEM-EDAX measurements were performed assessing the apatitic-type structure of the prepared films along with their satisfactory mechanical adhesion. Keywords: synthetic polymer, pulsed laser deposition, hydrodynamic model

Abstract: The polysiloxane networks have been prepared by means of hydrosilylation reaction of poly(dimethyl, hydrogen-methyl)siloxanes (HlPDMS) containing Si–H groups and siloxane copolymers named VlPDMS and VcPDMS containing Si-CH=CH2 functional groups. The structure of the siloxane copolymers was determined from 1H-NMR and FT-IR spectra analyse and the characterization of the siloxane copolymers was performed by gel permeation chromatography (GPC), viscosity and determination of the content of functional groups. The mechanical properties of the newly obtained networks were determined by physico-mechanical measurements, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). Keywords: polysiloxane, hydrosilylation crosslinking, polymer networks

Abstract: The objective of the present study was to check the debonding strength in four types of orthodontic adhesives frequently purchased at present in our country: Ortho-Loc, Transbond XT, Tranasbond Self Etching Primer and Grengloo. For this study forty patients with integral maxillary arches and minor dental-maxillary abnormalities were selected and further randomly divided into four groups (one for each adhesive), each group being bonded with Resolve® Classic Bracket-type metal brackets. After one month, during which the brackets were subjecte to the action of oral factors, they were debonded using the force measurement FK model of the SAUTER device. There were no signficant differences between the four types of adhesives, nevertheless, the highest values were obtained for Transbond XT. All four types of adhesives were within the clinically acceptable interval being able to be safely used by clinicians. Key words: orthodontic adhesives, brackets, self-etching, debonding strengths

Abstract: In this paper we determine a field of stress, which verifies the Cauchy equations of equilibrium, the conditions of continuity on the surfaces between layers, and boundary conditions, for a sandwich bar, with symmetrical distribution of layers, subject to traction. We have considered the relationships, previously obtained, for a composite bar built of three layers. Using a mediation formula for deformations and stress, we obtained a new formula for calculating the longitudinal elasticity modulus, in the case in which the constituent materials have different coefficients of transversal contraction. We realized the experimental measurements for test samples from polyesteric resin, reinforced with woven of fibers glass, carbon and glass-carbon. Keywords: composite materials, characteristic curve, elasticity modulus

Abstract: The major interest of practitioners for FRC dental restorations derives from the biological traits of this technique. The study concerned the biomechanical behaviour of polyethylene fiber reinforced composites. The experimental determination of the maximum pressure force that can be supported by a FRC fixed prosthetic restoration has also been targeted (Construct - Premise Indirect). For this purpose two samples have been used, each been made of a FRC restoration resin bonded to two lateral teeth which are fixed in a plaster block. The 2 samples differ by the fiber width and by the thickness of the veneer composite. The 2 samples have been tested with static short duration loads on a testing machine. It has been observed that both samples had a perfect elastic behaviour for force under 280N (sample 1) and 360N (sample 2). The maximum flexural stresses as a result of the load are 102.77MPa for the first pontic and 92.5MPa for the second one. The persistence of the fiber allows maintaining the bridge under certain conditions and for a certain period of time. If the stress increases the restoration is destructed (the force has to be almost doubled before the important displacement could lead to the fracture of the bridge). In a clinical situation this reversible and cost effective procedure, which is minimally invasive, aesthetically and biologically desirable offers a viable alternative to conventional replacement of lost teeth. Keywords: FRC, polyethylene fiber, prosthetic restoration

Abstract: This paper presents a numerical simulation for metal sheet bending with elastic pads made of silicone rubber reinforced with different ratios of very small glass fibers. Experimental tests were performed (traction and compression), in order to determine some characteristics for the materials and the results were used in the simulation program. A correction for the geometry of the punch, a prediction diagram for springback angle and the evolution of the force developed by the punch are also presented Keywords: numerical simultation, silicon rubber, glass fibers, finite elements

Abstract: The work presents the experimental determination of the transverse contraction coefficient í (Poissson’s ratio) for three kinds of composite materials. For this purpose, it was used the digital image correlation method that is an optical method for the measuring of the deformations. The composite materials were made of the same epoxy resin but the reinforcement was different: reinforcement with glass fabric; reinforcement with oak wood flour; reinforcement both with oak wood flour and with glass fabric (hybrid composite material). The specimens were subject to tensile test and during the test, it was used the system Aramis 2M for image acquisition and their post-processing to determine both the longitudinal strain ex (axial strain) on the direction of tensile loading and transversal strain ey on the direction of the specimen width. The variations of the transverse strains ey as a function of the axial ones for each specimen were graphycal lyrepresented. Then, the experimental data were approximated by linear regression that is by linear functions. The slopes of the straight lines represent Poisson’s ratio n. It was found that the adding of the oak wood flour as filling material in case of the composite materials reinforced with glass fabric, leads to the increasing of the Poisson’s ratio n . Keywords: composite material, wood flour, tensile test, Poisson’s ratio

Abstract: Aromatic polyimides containing side azobenzene groups have been synthesized by low-temperature solution polycondensation of certain aromatic dianhydrides with aromatic diamines containing preformed side azobenzene units followed by chemical imidization at 100°C in the presence of pyridine and acetic anhydride. The obtained polyimides were characterized and evaluated by FT-IR, UV-Vis, DSC and TGA analyses. These polymers show a good thermal stability with starting decomposition temperature above 300oC and glass transition temperature in the range of 111-230°C. These polyimides show a good solubility in various solvents. The photochromic behaviour, determined by the trans-cis isomerization process of azo-groups, in solution and in solid state, was also evaluated. Keywords: azopolyimides, photochromic behavior, thermal stability