Evaluation of the Vibration Signal during Milling Vertical Thin-Walled Structures from Aerospace Materials.

The main functions of thin-walled structures-widely used in several industries-are to reduce the weight of the finished product and to increase the rigidity of the structure. A popular method for machining such components, often with complex shapes, is using milling. However, milling involves undesirable phenomena. One of them is the occurrence of vibrations caused by the operation of moving parts. Vibrations strongly affect surface quality and also have a significant impact on tool wear. Cutting parameters, machining strategies and tools used in milling constitute some of the factors that influence the occurrence of vibrations. An additional difficulty in milling thin-walled structures is the reduced rigidity of the workpiece-which also affects vibration during machining. We have compared the vibration signal for different approaches to machining thin-walled components with vertical walls made of Ti6Al4V titanium alloy and Inconel 625 nickel alloy. A general-purpose cutting tool for machining any type of material was used along with tools for high-performance machining and high-speed machining adapted for titanium and nickel alloys. A comparison of results was made for a constant material removal rate. The Short-Time Fourier Transform (STFT) method provided the acceleration vibration spectrograms for individual samples.

Influence of Tools and Cutting Strategy on Milling Conditions and Quality of Horizontal Thin-Wall Structures of Titanium Alloy Ti6Al4V.

Titanium and nickel alloys are used in the creation of components exposed to harsh and variable operating conditions. Such components include thin-walled structures with a variety of shapes created using milling. The driving factors behind the use of thin-walled components include the desire to reduce the weight of the structures and reduce the costs, which can sometimes be achieved by reducing the machining time. This situation necessitates, among other things, the use of new machining methods and/or better machining parameters. The available tools, geometrically designed for different strategies, allow working with similar and improved cutting parameters (increased cutting speeds or higher feed rates) without jeopardizing the necessary quality of finished products. This approach causes undesirable phenomena, such as the appearance of vibrations during machining, which adversely affect the surface quality including the surface roughness. A search is underway for cutting parameters that will minimize the vibration while meeting the quality requirements. Therefore, researching and evaluating the impact of cutting conditions are justified and common in scientific studies. In our work, we have focused on the quality characteristics of horizontal thin-walled structures from Ti6Al4V titanium alloys obtained in the milling process. Our experiments were conducted under controlled cutting conditions at a constant value of the material removal rate (2.03 cm3/min), while an increased value of the cut layer was used and tested for use in finishing machining. We used three different cutting tools, namely, one for general purpose machining, one for high-performance machining, and one for high-speed machining. Two strategies were adopted: adaptive face milling and adaptive cylindrical milling. The output quantities included the results of acceleration vibration amplitudes, and selected surface topography parameters of waviness (Wa and Wz) and roughness (Ra and Rz). The lowest values of the pertinent quantities were found for a sample machined with a high-performance tool using adaptive face milling. Surfaces typical of chatter vibrations were seen for all samples.

Tribological properties of polypropylene composites with carbon nanotubes and sepiolite.

Carbon nanotubes (CNTs) and sepiolite (SEP) were modified in order to improve their compatibility with the polypropylene (PP) matrix. Carboxylic groups were introduced into the CNTs through an oxidative treatment and aliphatic chains were incorporated on SEP by ion exchange of a cationic surfactant. Maleic anhydride grafted polypropylene (PPgMA) was mixed with neat PP to introduce polar groups into the polymer matrix. Composites including modified and non-modified fillers were prepared by melt extrusion. Dispersion and interaction of the CNTs with the PP and PPgMA matrices were evaluated by Raman spectroscopy while a focused ion beam/scanning electron microscopy (FIB/SEM) was used for SEP containing composites. Scratch resistance, microhardness, dynamic friction and wear were determined. Raman spectroscopy shows that the introduction of polar groups into PP matrices has a positive effect on the dispersion of modified CNTs. FIB/SEM results show that the modification of SEP improves its dispersion in the polypropylene matrix; filler clusters found in the PPgMA matrix are much times smaller than those in the neat PP. Despite of SEP agglomerates in the composites, a good interaction between both phases is seen; SEP particles are fully coated and embedded inside the PP matrix. The 'lack of cooperation' between unmodified PP and its fillers results in nanocomposites with larger residual depths; by contrast, PPgMA does 'cooperate' with its fillers so that the nanocomposites in scratch resistance testing have smaller residual depths R(h) than the neat PPgMA. Addition of the fillers to PPgMA also increases the hardness. As for wear rates, some our fillers provide higher and some lower wear rates than PP.

Scratch and abrasion properties of polyurethane-based micro- and nano-hybrid obturation materials.

Polyurethane-based micro- and nano-hybrid composites were produced with controlled porosity to be used as obturation materials. In addition to hydroxyapatite (HAp) micro-particles in the composites, two different ceramics particle types were also added: alumina micro-particles and silica nano-particles. Particles of different sizes provide the materials with improved mechanical properties: the use of micro- and nano-particles produces a better packing because the nano-particles fill the interstitial space left by the micro-particles, rendering an improvement in the mechanical properties. The silica and alumina particles provide the materials with appropriate abrasion and scratching properties, while the HAp provides the required bio-acceptance. The polymeric matrix was a mono-component solvent-free polyurethane. The porosity was selected by controlling the chemical reaction.

Tribological properties of polymer nanohybrids containing gold nanoparticles obtained by laser ablation.

We have studied polystyrene (PS)+Au particles nanohybrids. Approximately spherical gold nanoparticles with the average diameter of 15 nm were obtained by laser ablation in a liquid environment. Thus any chemical residue on the particles was eliminated. Focused ion beam (FIB) milling plus scanning electron microscopy (SEM) observation show that Au particles are fairly well dispersed inside the polymer matrix, better than when PS is simply dissolved in a nanoparticle solution. The Au particles concentration as low as 0.15 wt% results in dramatic changes in tribological properties, namely dynamic friction and pin-on-disk wear. Both wear and dynamic friction results are explained in terms of high brittleness of PS, abrasion of Au particles against a ceramic indenter, and also effects of density of filler particles in the matrix on tribological properties. Effects of varying normal load on friction are small.

Optimization of tribological and mechanical properties of nanocomposites of polyurethane/poly(vinyl acetate)/CaCO3.

Properties of samples containing polyurethane (PU), poly(vinyl acetate) (PVAc) and nanosize particles of calcium carbonate (CaCO3) are correlated with concentrations of these components. Interphase phenomena in PU/PVAc/CaCO3 nanohybrids have been studied before, we focus here on wear and scratch resistance. In addition to polymer blends containing CaCO3, the effects of adding CaCO3 with grafted PVAc, and CaCO3 with grafted silane and PVAc in varying ratios are also evaluated. For blends that do not contain the filler, a hypothesis explaining the concentration dependence of friction called the Bump Model is advanced and supported by the experimental results. In particular, we explain how creating a blend containing only 10% of a second polymer results in a dramatic drop of friction of the majority polymer. In single scratch testing, above 3% the filler displays 'its own' resistance to scratching. Chemical modification of the filler results in shallower residual depths--a consequence of improved interaction of the filler with the polymeric matrix. In sliding wear determination, strain hardening is seen for blends as well as for filler-containing composites. In tensile testing, addition of an unmodified filler increases the elongation at break and thus lowers the brittleness; the effect is even larger for chemically modified fillers.

Vulcanization characteristics and dynamic mechanical behavior of natural rubber reinforced with silane modified silica.

Two silane coupling agents were used for hydrolysis-condensation reaction modification of nanosilica surfaces. The surface characteristics were analyzed using Fourier transform infrared spectroscopy (FTIR). The vulcanization kinetics of natural rubber (NR) + silica composites was studied and compared to behavior of the neat NR using differential scanning calorimetry (DSC) in the dynamic scan mode. Dynamic mechanical analysis (DMA) was performed to evaluate the effects of the surface modification. Activation energy E(a) values for the reaction are obtained. The presence of silica, modified or otherwise, inhibits the vulcanization reaction of NR. The neat silica containing system has the lowest cure rate index and the highest activation energy for the vulcanization reaction. The coupling agent with longer chains causes more swelling and moves the glass transition temperature T(g) downwards. Below the glass transition region, silica causes a lowering of the dynamic storage modulus G', a result of hindering the cure reaction. Above the glass transition, silica-again modified or otherwise-provides the expected reinforcement effect.

Natural-synthetic hybrid polymers developed via electrospinning: the effect of PET in chitosan/starch system.

Chitosan is an amino polysaccharide found in nature, which is biodegradable, nontoxic and biocompatible. It has versatile features and can be used in a variety of applications including films, packaging, and also in medical surgery. Recently a possibility to diversify chitosan properties has emerged by combining it with synthetic materials to produce novel natural-synthetic hybrid polymers. We have studied structural and thermophysical properties of chitosan + starch + poly(ethylene terephthalate) (Ch + S + PET) fibers developed via electrospinning. Properties of these hybrids polymers are compared with extant chitosan containing hybrids synthesized by electrospinning. Molecular interactions and orientation in the fibers are analyzed by infrared and Raman spectroscopies respectively, morphology by scanning electron microscopy and thermophysical properties by thermogravimetric analysis and differential scanning calorimetry. Addition of PET to Ch + S systems results in improved thermal stability at elevated temperatures.

Wear resistance and wear mechanisms in polymer + metal composites.

We have investigated composites containing metallic micro-size and nano-sized particles as the 10 wt% dispersed phase. Branched low density polyethylene (LDPE) was the matrix. Microsized metals were Al, Ag and Ni; nanosized metals were Al and Ag. Several mechanisms of wear are observed in function of the kind and size of metal used: deformation, delamination, abrasion, adhesion and rolls formation. The presence of Ag particles increases the wear rate as compared to neat LDPE. The presence of Al particles lowers the wear of LDPE significantly; nanoparticles are more effective than microparticles.

Scratch resistance of polycarbonate containing ZnO nanoparticles: effects of sliding direction.

Abrasive wear resistance of injection molded polycarbonate (PC) and polycarbonate + zinc oxide nanocomposites containing 0.5 wt% ZnO nanoparticles was determined as a function of the sliding direction with respect to injection flow. First we have performed single scratch testing under progressively increasing the load applied. Then sliding wear testing consisting of 15 successive scratches along the same groove was performed. Neat PC shows anisotropic behavior, with instantaneous penetration depth more than 50% higher in the direction parallel to the melt injection flow than in the transverse direction. Viscoelastic recovery after scratching of neat PC is also higher in the longitudinal than in the transverse direction, hence final residual depth values are similar in both directions. The addition of ZnO nanoparticles reduces the instantaneous penetration depth in the longitudinal direction but lowers viscoelastic recovery so that the residual depth is large. In the transverse direction, the scratch resistance is similar for neat PC and the nanocomposite. Dynamic mechanical analysis, SEM/FIB results and wear mechanisms from SEM observations of wear scars are discussed. Below the glass transition region the nanocomposite has distinctly higher storage modulus E' than PC--a clear reinforcement effect. However, the addition of ZnO nanoparticles to the polymer increases the material brittleness at room temperature by a factor of 2.72.

Effects of UV Stabilizers on Polypropylene Outdoors.

Hindered amine light stabilizers (HALSs) and nano ZnO were used to stabilize polypropylene (PP) film-based formulations that were exposed to ultraviolet (UV) light for different lengths of time, simulating the harsh outdoor weather of Dallas, Texas, USA. UV doses applied in our laboratory are 121 times larger than the UV dose provided by the sunlight in Texas. 15 different compositions were studied. Tensile behavior, UV transmittance, thermal stability (by thermogravimetric analysis) and dynamic friction of the so exposed PP-based films were determined. Scanning electron micrographs of fracture surfaces were obtained. Nano-ZnO-containing stabilizers impart strong UV resistance to our films. The combination of HALSs and nano-ZnO stabilizers makes the PP films harder-which is important for some PP applications, such as toy manufacturing.

Tribological properties of epoxy + silica hybrid materials.

Various amounts of nano size silica particles prepared by a sol-gel process were added to epoxy+ amine systems. We have investigated tribological properties including friction and sliding wear resistance of hybrids so obtained, and also relationships between different tribological properties and surface topography. The thermal degradation behavior and thermal stabilities were determined by thermogravimetric analysis (TGA). The introduction of silica into bisphenol-A type epoxy resins does not affect significantly char formation in the epoxy resins; relatively small improvements in thermal stabilities is seen. At the same time, results from a pin-on-disc tribometer show that silica addition causes lowering of friction already at 1 part per hundred (phr) and very significant lowering of wear at 2 or more phr. We have found finer waves of the worn surface in the hybrids than in the neat epoxy. SEM results demonstrate that the silica particles improve the wear resistance by hindering crack propagation.

Reinforcement of polymeric latexes by in situ polymerization.

Two silicas with different particle sizes have been synthesized by the Stöber method. The particles have been functionalized with methacryloyl groups. In situ emulsion polymerization of butyl acrylate and methyl methacrylate in the presence of functionalized silica particles was performed. The ratio of butyl acrylate to methyl methacrylate was varied in order to optimize the composition for improvement of tribological and thermophysical properties. The silica particles morphology and functionalization have been determined respectively by scanning electronic microscopy and infrared spectroscopy. The composites were characterized also by thermogravimetric analysis, differential scanning calorimetry, microscratch testing and static light scattering. The latex reinforced with the smallest functionalized silica exhibits higher thermal stability than the non reinforced latex, along with lower penetration depth and higher residual depth in progressive load scratch testing. Thus, the resistance to penetration is increased while viscoelastic healing is hampered by silica particles.

Scratch and wear resistance of polyamide 6 reinforced with multiwall carbon nanotubes.

While carbon nanotubes have been used for a variety of purposes, it was not known whether they can improve tribological properties of polymers. Polyamide 6 (PA6) has been reinforced with 0.2, 0.5 and 1.0 wt% of multiwall carbon nanotubes (MWCNTs) by melt mixing process and characterized by scanning electron microscopy (SEM), transmission electron microscopy, thermogravimetric analysis (TGA), scratching, sliding wear and tensile testing. TGA results for the air atmosphere show that MWCNTs shift the onset of thermal degradation to higher temperatures. Sliding wear tests show that the penetration depth decreases as the concentration of carbon nanotubes increases. However, the viscoelastic healing is hampered by the MWCNTs presence and the residual depths increase at the same time. Narrower scratch groove widths are seen in SEM for composites with MWCNTs, and scratch hardness increases. Tensile tests show an increase of 27% in the Young modulus value upon addition of 1.0% of MWCNTs. The stress at yield is also higher for the nanocomposites.

Nanoscale confinement effects on the relaxation dynamics in networks of diglycidyl ether of bisphenol-A and low-molecular-weight poly(ethylene oxide).

Thermoplastic poly(ethylene oxide) (PEO) (Mw(PEO) approximately 4000) has been used to prepare thermosetting nanocomposites incorporating diglycidyl ether of bisphenol A (DGEBA) epoxy oligomer. Blends with various PEO/DGEBA weight ratios were cured using stoichiometric portions of 4,4'-diaminodiphenylmethane. The resulting semi-interpenetrating polymer networks were studied by several techniques. Nanoscale confinement effects, thermal (glass transition, melting and crystallization temperatures) and structural features of our materials are similar to those for networks with much higher Mw(PEO) and different curing agents; however, the polyether crystallization onset occurs in our case at a lower PEO concentration; shorter PEO chains organize themselves more easily into crystalline domains. Very low estimates of the k parameter of the Gordon-Taylor equation, used to fit the compositional dependences of the dielectric and calorimetric glass transition temperatures, and a strong plasticization of the motion of the glyceryl segments (beta-relaxation) in the epoxy resin were observed. These illustrate an intensified weakening in the strength of the intermolecular interactions in the modified networks, as compared to the high strength of the self-association of hydroxyls in the neat resin. The significance of hydrogen-bonding interactions between the components for obtaining structurally homogeneous thermoset-i-thermoplastic networks is discussed.

Improvement of Scratch and Wear Resistance of Polymers by Fillers Including Nanofillers.

Polymers have lower resistance to scratching and wear than metals. Liquid lubricants work well for metals but not for polymers nor for polymer-based composites (PBCs). We review approaches for improvement of tribological properties of polymers based on inclusion of fillers. The fillers can be metallic or ceramic-with obvious consequences for electrical resistivity of the composites. Distinctions between effectiveness of micro- versus nano-particles are analyzed. For example, aluminum nanoparticles as filler are more effective for property improvement than microparticles at the same overall volumetric concentration. Prevention of local agglomeration of filler particles is discussed along with a technique to verify the prevention.

Synthesis of silver nanoparticles using aqueous extracts of Heterotheca inuloides as reducing agent and natural fibers as templates: Agave lechuguilla and silk.

Silver nanoparticles (Ag NPs) were synthesized using a one-pot green methodology with aqueous extract of Heterotheca inuloides as a reducing agent, and the support of natural fibers: Agave lechuguilla and silk. UV-Vis spectroscopy, X-Ray photoelectron spectroscopy XPS and transmission electron microscopy TEM were used to characterize the resulting bionanocomposite fibers. The average size of the Ag NPs was 16nm and they exhibited low polydispersity. XPS studies revealed the presence of only metallic Ag in the nanoparticles embedded in Agave. lechuguilla fibers. Significant antibacterial activities against gram-negative Escherichia coli and gram-positive Staphylococcus aureus were determined. AgO as well as metallic Ag phases were detected when silk threads were used as a substrates hinting at the active role of substrate during the nucleation and growth of Ag NPs. These bionanocomposites have excellent mechanical properties in tension which in addition to the antibacterial properties indicate the potential use of these modified natural fibers in surgical and biomedical applications.

Rheological Characterization of Liquid Polymers Containing Ceramic Nanopowders for Use in Thermoelectric Devices.

We have determined shear viscosities as a function of temperature for several liquid high temperature polymers (HTPs) as potential coatings for solid state thermoelectric generators (TEGs) as well as for TE coolers (TECs). To each HTP we added in turn several ceramic nanopowders: alumina, silica and multi-wall carbon nanotubes (MWCNTs). The shear rate applied range is from 0.0002 to 60 s(-1). The results are compared to those for neat HTPs. For a given HTP, we obtain for some nanopowders significant lowering of viscosity, or else a significant increase, or else a small effect only. Possible reasons for such differences in behavior are discussed in terms of the spatial structures of CNTs (random orientations at low temperatures), and the interactions between functional groups on HTPs and atoms in the nanoceramics.

Synthesis, characterization, and in vitro evaluation of gamma radiationinduced PEGylated isoniazid

Background: The search for innovative anti-tubercular agents has received increasing attention in tuberculosis chemotherapy because Mycobacterium tuberculosis infection has steadily increased over the years. This underlines the necessity for new methods of preparation for polymer-drug adducts to treat this important infectious disease. The use of poly(ethylene glycol)(PEG) is an alternative producing anti-tubercular derivatives. However, it is not yet known whether PEGylated isonicotinylhydrazide conjugates obtained by direct links with PEG are useful for therapeutic applications. Results: Here, we synthesized a PEGylated isoniazid (PEG-g-INH or PEG­INH) by gamma radiation-induced polymerization, for the first time. The new prodrugs were characterized using Raman and UV/Vis spectrometry. The mechanism of PEGylated INH synthesis was proposed. The in vitro evaluation of a PEGylated isonicotinylhydrazide macromolecular prodrug was also carried out. The results indicated that PEG­INH inhibited the bacterial growth above 95% as compared with INH, which showed a lower value (80%) at a concentration of 0.25 µM. Similar trends are observed for 0.1, 1, and 5 µM. Conclusions: In summary, the research suggests that it is possible to covalently attach the PEG onto INH by the proposed method and to obtain a slow-acting isoniazid derivative with little toxicity in vitro and higher antimycobacterial potency than the neat drug.

Poly(3-hydroxybutyrate) graft copolymer dense membranes for human mesenchymal stem cell growth

Background: The use of novel materials as an artificial extracellular matrix for stem cell growth is a current strategy of increasing interest for regenerative medicine. Here, we prepare thermal-remolded membrane scaffolds from poly(3-hydroxybutyrate) grafted with 2-amino-ethyl methacrylate hydrochloride. However, it is unclear whether these membranes are useful for tissue engineering. Results: The mechanical properties, tribology, and morphology of the dense membranes were assessed. The results show that tensile strain at break and roughness of the compressed membrane decrease with increasing graft degree. Moreover, graft copolymer membranes showed lower resistance to scratching, greater degree of swelling and higher brittleness than un-grafted P(3HB) films. Thus, it effectively supports the growth of dermal fibroblast, as demonstrated by epifluorescence microscopy. Conclusions: It is concluded that the developed membrane can be properly used in is the restoration of skin tissue. How to cite: González-Torres M, Sánchez-Sánchez R, Solís-Rosales SG, et al. Poly(3-hydroxybutyrate) graft copolymer dense membranes for human mesenchymal stem cell growth.