Key Engineering Materials Vol. 699

Abstract: In this study, multilayered composite plates with shape memory properties were produced: carbon fibers prepreg are alternated with layers of shape memory epoxy powder obtaining composite with different number of layers. The differences in the load exerted during shape recovery, and percentage and time of recovery of the composites as a function of layers number have been evaluated. In particular, the actuation load and the shape recovery percentage were measured after a V-shape memorizing step of the composites. The experimental results are very promising, showing that such multilayers can successfully recover the original shape without noticeable damages and an increasing of actuation load per layer has been found at the increase of the layers number.

Abstract: The objective of this research is to investigate the effect of the annealing process on the mechanical and electrical properties of thermoplastic polyurethane/multi-walled carbon nanotube (TPU/MWCNT) nanocomposites. The TPU/MWCNT nanocomposites were prepared by melt-mixing the TPU with 1, 3, and 5 wt.% MWCNT. The TPU/MWCNT injection-molded specimens were annealed at 110°C at different annealing time intervals. The mechanical properties of TPU/MWCNT nanocomposites (Young modulus, tensile strength at yield, stress at break and strain at break) were measured before and after the annealing at constant crosshead speed of 100 mm/min. Furthermore, the DC conductivity of the TPU/MWCNT injection molding specimens was measured before and after annealing, and the percolation thresholds were determined based on following the percolation theory.

Abstract: Carbon nanotubes offer the possibility of substantial improvements in the properties of polymer-based composites. However, adding carbon nanotubes increases the viscosity and makes the composites more difficult to process. Consequently, understanding the rheological behavior of nanocomposites is important from both the theoretical and industrial points of view. In the present work, rheological behavior of thermoplastic polyurethane filled with various amounts (1, 3 and 5 wt.%) of multi-walled carbon nanotubes was investigated by capillary rheometry. In this regard, the melt flow behavior of the nanocomposite was measured using a capillary rheometer with a die length-diameter ratio of 30:1, 20:1 and 10:1. In order to investigate the effect of temperature on viscosity, the tests were carried out in the temperature range of 180 to 210°C. The shear rate examined between 100 and 5000 s^-1, cover the shear experienced during most polymer processing techniques. The Bagley and Weissenberg-Rabinowitsch correction was performed to determine the real viscosity of the nanocomposites; moreover, the Cross viscosity model coefficients were determined.

Abstract: The paper presents the extruder functioning point changing, resulted as the intersection of screw and extrusion head functional characteristics curves, during the processing of a polymeric material recycled many times, in comparison with the same polymeric material - first time extruded. The recycling could modify the slope of the screw characteristic curve. In the paper it is shown the modification of the screw characteristic, at the same screw speed, for a sort of HDPE, at a certain extruding temperature. Similar, there are modifications of the extruder head characteristics curves. So the extruder functioning point becomes quite different as the number of recycling increases, but it has to be situated in the head optimal working field.

Abstract: The paper addresses the study of the damping characteristics estimation and behaviour of the magnetorheological elastomers (MREs) in the absence of magnetic field. This type of material actively changes the size, internal structure and viscoelastic characteristics under the external influences. These particular composite materials whose characteristics can vary in the presence of a magnetic fields are known as smart materials. The feature which causes the variation of properties in magnetic fields is explained by the existence of polarized particles which change the material form by energy absorbing. Damping is a special characteristic that influences the vibratory of the mechanical system. As an effect of this property is the reducing of the vibration amplitudes by dissipating the energy stored during the vibratory moving. The main characteristic that is based on the determination of the damping coefficient is the energy loss, which is the subject of the present paper. Before to start the characteristics determination in the presence of the magnetic field, it is necessary to study these characteristics in the absence of magnetic field. The MRE specimens have been manufactured and tested under the light conditions (non magnetic field). A special experimental test rig was built to investigate the response of the MRE specimens under the charging force. The experimental results show that the loss energy of the MRE specimen can be determined from the charging-discharging curves versus displacement. The results of the MRE specimen are presented in this paper: MRE with feromagnetic particles not exposed in magnetic field during fabrication.

Abstract: Radiation crosslinking of polyamidu 6 (PA 6) is a well-recognized modification of improving basic material characteristics. Radiation, which penetrated through specimens and reacted with the cross-linking agent, gradually formed cross-linking (3D net), first in the surface layer and then in the total volume, which resulted in considerable changes in specimen behaviour. This research paper deals with the possible utilization of irradiated PA6. The material already contained a special cross-linking agent TAIC (5 volume %), which should enable subsequent cross-linking by ionizing β – radiation (15, 30 and 45 kGy). The effect of the irradiation on mechanical behavior of the tested PA 6 was investigated. Material properties created by β – radiation are measured by nanoindentation test using the DSI method (Depth Sensing Indentation). Hardness increased with increasing dose of irradiation at everything samples; however results of nanoindentation test shows increasing in nanomechanical properties of surface layer. The highest values of nanomechanical properties were reached radiation dose of 45 kGy, when the nanomechanical values increased by about 95%. These results indicate advantage cross-linking of the improved mechanical properties.

Abstract: The process of radiation crosslinking helps to improve some mechanical properties of polymer materials. Micro-mechanical changes in the surface layer of glass-fiber filled PA 66 modified by beta radiation were measured by the Depth Sensing Indentation - DSI method on samples which were non-irradiated and irradiated by different doses of the β - radiation. The specimens were prepared by injection technology and subjected to radiation doses of 0, 33, 66 nad 99 kGy. The change of micro-mechanical properties is greatly manifested mainly in the surface layer of the modified polypropylene where a significant growth of micro-hardness values can be observed. Indentation modulus increased from 1.8 to 3.0 GPa (increasing about 66%) and indentation hardness increased from 87 to 157 MPa (increasing about 80%). This research paper studies the influence of the dose of irradiation on the micro-mechanical properties of semi-crystalline polyamide 66 filled by 30% glass fiber at room temperature. The study is carried out due to the ever-growing employment of this type of polymer.

Abstract: Surface micropatterning of polymers is an important process in a large number of applications ranging from microelectronics, sensors design and material science, to tissue engineering and cell biology. In this study a simple and versatile method for manufacturing micro-scale polymer surface patterns has been developed. Micropatterned surfaces of acrylonitrile-butadiene–styrene (ABS) were engineered by compression molding. Two different micropatterned surfaces were fabricated using diverse molds. The first micropatterning was achieved on a brass mold by the intersection of instrumented microindentation traces. The second microsculptured surface was realized through a bronze sintered mold. The morphological aspects and the surface wettability after microsculpturing were investigated. The microsculptured ABS surface produced by the sintered mold shows a higher contact angle compared with those of flat ABS surfaces. From the experimental results, it was found that the intrinsic hydrophobicity of the material is enhanced simply through increasing surface roughness of the solid surface. The method presented is an economical process to fabricate hydrophobic microsculptured surfaces and it is suitable for many kinds of materials.

Abstract: The aim of this work was to obtain hybrid Co/UHMWPE composite biocoatings reinforced by UHMWPE (ultra high molecular weight polyethylene) biopolymer microparticles in the cobalt matrix, by electro-codeposition technique, with possibilities to use them as biomaterials. UHMWPE was selected as surface modifier element, due to its high biocompatibility and low coefficient of friction being used in many biomedical applications. Cobalt is already used in biomedical implants as cobalt – chromium alloys. The obtained coatings were investigated in terms of surface morphology (scanning electron microscopy - SEM), chemical composition and inclusion percentage (energy dispersive X-ray spectroscopy - EDX), roughness and microtopography (atomic force microscopy - AFM), coating thickness and microhardness. The SEM morphologies of electrodeposited pure cobalt and Co/UHMWPE composite biocoatings, show differences due to UHMWPE biopolymer particles incorporation in the cobalt matrix. The inclusion of UHMWPE microparticles increases with increasing the UHMWPE concentration in the electrolyte as was demonstrated by EDX investigations. The addition of the UHMWPE biopolymer microparticles to the deposition bath led to an increase of the roughness of hybrid coatings comparatively with pure cobalt coating obtained under the same conditions. The coating thickness of the electroplated surfaces as were observed by cross sectional scanning electron micrographs confirm higher adhesion strength of the coatings on the stainless steel support.

Abstract: The present work has the purpose to study and to evaluate the corrosion resistance of zinc layers and phenol – formaldehyde resin/Zn composite layers obtained by electrodeposition. For the best results it was used different parameters for electrodeposition such as: current density between 3 – 5 A/dm², time for electrodeposition: 30 minutes and 60 minutes, stirring rate: 500 rpm and 800 rpm. Different sizes (mean diameter size between 0.1 – 5.0μm and 6.0 – 10.0μm) of dispersed phases were used with concentration into electrolyte solution from 10g/L to 25 g/L of polymer particles. The morphology of the layers was investigated by SEM – EDX methods. The surface morphology of composite layers was different as compared with pure zinc layers. By adding polymer particles into zinc electrolyte during electroplating a very good distribution of polymer on zinc layer surface was obtained. The electrochemical behavior of the composite layers in the corrosive environment was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solution 0.5M sodium chloride was used in a three electrode open cell.