Ulku Yilmazer

Slip Effects in Capillary and Parallel Disk Torsional Flows of Highly Filled Suspensions

The shear viscosity material function of a highly filled suspension consisting of a Newtonian poly(butadiene acrylonitrile acrylic acid terpolymer) matrix, PBAN, mixed with an ammonium sulfate filler at 60% by volume was studied. Both capillary and parallel disk torsional flows were employed. The rheological characterization revealed strong slip of the suspension at the walls over a broad range of shear stresses in both types of flows. The slip velocity increased approximately linearly with the shear stress. In capillary flows, above a critical shear stress, flow took place in a pluglike manner, owing to slip at the wall. The experimental findings were further elucidated to determine the slip layer thickness and the apparent shear viscosity behavior of highly filled suspensions at high shear stress at the wall values. It was concluded that the slip effects dominate the flow of highly filled suspensions and the true flow and deformation characteristics of the highly filled suspensions may be overshadowed b...

Rheological behavior of a concentrated suspension: A solid rocket fuel simulant

The rheological behavior of a very concentrated suspension (76.5 vol %), which serves as a widely used solid rocket fuel simulant, was characterized employing both torsional and capillary flows. No comprehensive studies of the rheology of concentrated suspensions have been carried out previously at such a high solids content. The suspension exhibited shear thinning over the apparent shear rate range of 30–3000 s−1. Significant slip at the wall was observed in both torsional and capillary flows with the slip velocity increasing from about 0.001 mm/s at a shear stress of 4 Pa to as high as 60 mm/s at 100 kPa. A flow visualization technique was applied for the first time to determine the wall slip velocities in torsional flow directly, to also provide the true deformation rate and feedback on yielding. The contribution of the slip of the suspension at the wall to the volumetric flow rate in capillary flow was found to increase with decreasing shear stress, giving rise to plug flow at sufficiently low shear s...

Some microwave and mechanical properties of carbon fiber-polypropylene and carbon black-polypropylene composites

The insertion loss, return loss and electromagnetic shielding effectiveness of carbon-black and carbon-fiber-filled polypropylene composites were studied in the 8 to 12 GHz frequency range. Both electromagnetic and mechanical measurements were performed at various filler concentrations. The maximum shielding values obtained for carbon-fiber-filled composites was over 40 dB for 35 wt% concentration. Youngs modulus, ultimate strength and elongation at break with respect to filler concentration were also determined. The stiffness and brittleness of the composites increased with increasing filler concentration. The ultimate strength of the carbon-black-filled composites increased whereas the ultimate strength of carbon-fiber-filled composites decreased with the filler content. Elongation at break values for all composites dropped sharply with the filler concentration.

Effect of fillers on thermal and mechanical properties of polyurethane elastomer

The effects of five different types of fillers on the thermal and mechanical properties of hydroxyl-terminated polybutadiene-based polyurethane elastomers were explored to develop a filled polyurethane elastomeric liner for rocket motors with hy- droxyl-terminated polybutadiene-based composite propellants. Two types of carbon black, silica, aluminum oxide, and zirconium(III) oxide were used as filler. Based on the improvement in the tensile properties and the erosion resistance achieved in the first part of the study, an ISAF-type carbon black was selected to be used as the main filler in combination with an additional filler. The second part involves the investigation of polyurethane elastomers containing a second filler in various amounts in addition to the ISAF-type carbon black used as the main filler. In addition to the thermal and mechanical properties, the processability of the uncured polyurethane mixtures were also explored by measuring the viscosity in this second part of the study. The studied fillers do not considerably change the thermal degradation temperatures and the ther- mal conductivity of the polyurethane elastomers with a filler content up to 16 wt %. The best improvement in the erosion resistance and tensile strength of the polyurethane elastomers with additional fillers is also achieved when filled with the ISAF-type carbon black, whereas the use of zirconium(III) oxide as additional filler provides almost no improvement in these properties. Viscosity of the uncured polyurethane mixtures increases with the increasing filler content and with the decreasing particle size of the filler. Aluminum oxide-filled elastomers seem to be the most suitable compositions having sufficiently high thermal and mechanical properties, together with the process- ability of uncured mixtures. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1057-1065, 1998

Viscoelastic characteristics of chain extended/branched and linear polyethylene terephthalate resins

Two chemically modified chain extended/branched polyethylene terephthalate (PET) resins and one unmodified resin, considered to be linear, were characterized in terms of their melt flow, die swell, and viscoelastic properties. The three resins had reportedly similar nominal intrinsic viscosities but exhibited different viscoelastic behavior. The modified resins had lower melt flow index, higher die swell, higher complex viscosity and higher storage modulus than the unmodified one. The Cole–Cole plots of the resins were independent of temperature, and the data for modified resins formed a group that lay below the data group for the unmodified PET. The distribution of relaxation times was determined. The modified resins had higher relaxation strength, Gi, especially at high relaxation times, λi. The mean relaxation times of the chain extended/branched resins were approximately an order of magnitude higher than that of the unmodified resin, implying pronounced elastic character. The modified resins had better foaming characteristics in extrusion foam processing than the unmodified one owing to their elastic nature.

Tensile, flexural and impact properties of a thermoplastic matrix reinforced by glass fiber and glass bead hybrids

Abstract The tensile and flexural properties, together with the unnotched and notched impact strengths of acrylonitrile butadiene styrene (ABS), glass bead (GB) and glass fiber (GF) ternary composites have been studied. The tensile strengths of the hybrid composites were predicted, from a theory formulated in this study, by using the strengths of the ABS/GB and ABS/GF composites alone. The addition of even a small quantity of glass fiber to ABS or ABS/GB composites changes the mode of deformation from one which shows vacuole growth and crazing to brittle failure. Thus, the elongation at break, the area under the stress/strain curve and the impact energy decrease sharply with the introduction of glass fiber to ABS or ABS/GB composites.

Mechanical and burning properties of highly loaded composite propellants

An improvement in the performance of solid rocket motors was achieved by increasing the oxidizer content of HTPB-based solid propellants. To minimize the adverse changes in the mechanical and rheological properties due to the increased amount of hard solid particles in the soft polymeric binder matrix, the optimum combi- nation of the particle sizes and volume fractions of the bimodal ammonium perchlorate and the aluminum powder in the solid load was obtained from the results of testing a series of propellant samples prepared by using ammonium perchlorate in four different average particle sizes, 9.22, 31.4, 171, and 323 mm. The maximum packing density of solids in the binder matrix was determined by changing the sizes and the volume fractions of fine and coarse ammonium perchlorate at constant solid loading. The aver- age size (10.4 mm) and concentration of aluminum powder used as metallic fuel were maintained constant for ballistic requirements. Optimum sizes and fine-to-coarse ratio of ammonium perchlorate particles were determined to be at mean diameters of 31.4 and 323 mm and fine-to-coarse ratio of 35/65. Solid content of the propellant was then increased from 75 to 85.6% by volume by using the predetermined optimum sizes and fine to coarse ratio of ammonium perchlorate. Mechanical properties of the propellant samples were measured by using an Instron tester with a crosshead speed of 50 mm/ min at 257C. The effect of oxidizer content and fine-to-coarse ratio of oxidizer on the burning rate of the propellant was also investigated by using a strand burner at various pressures. From experiments in which the size and the fine-to-coarse ratio of ammo- nium perchlorate were changed at constant solid loading, a minimum value of initial modulus was obtained for each fine-to-coarse ratio, indicating that the solids packing fraction is maximum at this ratio. The tensile strength and the burning rate increase, while the elongation at maximum stress decreases with increasing fine-to-coarse ratio of ammonium perchlorate. Experiments in which the total solid loading was increased at constant fine-to-coarse ratio of ammonium perchlorate show that the modulus, the tensile strength and the burning rate increase, while the elongation at maximum stress decreases with increasing solid loading. Propellants having solid loading of up to 82% exhibit acceptable mechanical properties and improved burning properties suitable for rocket applications. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 1457-1464, 1998

Properties and applications of sandwich panels based on PET foams

PET foams of variable densities (1 g/cc to 0.2 g/cc), based on virgin and recycled material were produced by extrusion with physical or chemical blowing agents and evaluated as low density core in sandwich panels having M/F impregnated paper or flame retardant mineral reinforced PET as skin faces. Flexural and shear stiffness of the laminates were determined by variable span three point bending. Panels were also tested for thermal and moisture stability and compared with competitive sandwich constructions based on PVC foam, flake board, particleboard and plywood. Potential applications of the PET based laminates in building and construction are presented.

Modification and characterization of bentonite with quaternary ammonium and phosphonium salts and its use in polypropylene nanocomposites

Montmorillonite-rich local bentonite was modified with four different quaternary alkyl salts: hexadecyltrimethylammonium bromide ((HMA)(Br)), tetra(kis)decylammonium bromide ((TKA)(Br)), tetrabutylammonium tetrafluoroborate ((TBA)(BF4)), and tetrabutylphosphonium tetrafluoroborate ((TBP)(BF4)) to produce organoclays. The organoclays produced were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and x-ray diffraction (XRD). The XRD results indicate that the d-spacing of the organoclays increased as a consequence of the exchange of Na+ ions in the clay galleries with the cation of the surfactants with long alkyl tails. The d-spacing of the bentonite increased from 1.2 nm to 1.78 nm, 2.56 nm, 1.48 nm, and 1.64 nm after modification with HMA, TKA, TBA, and TBP cations, respectively. TGA analysis of the organoclays showed that the decomposition temperatures of the organoclays were higher than the melt processing temperature of polypropylene (PP), permitting the use of these organoclays in melt processing of PP. Ternary composites of PP/PP-grafted maleic anhydride/organoclays were prepared using a twin screw extruder followed by injection molding for characterization. Transmission electron microscopy analysis of the composites showed intercalated structures as well as microcomposite formations. Mechanical properties of pure PP were improved through ternary composite formation.

Mechanical Properties of Unsaturated Polyester / Montmorillonite Composites

Abstract : Nanocomposites composed of unsaturated polyester matrix and organically modified clay filler were prepared. After the synthesis, XRD patterns showed that the interlayer spacing expanded from 1.25 nm to 4.5 nm. The mechanical properties of the nanocomposites were determined and it was found that adding only 3 w/w% organically modified clay improved the flexural modulus of unsaturated polyester by 35%. From DSC diagrams, it was found that T(sub g) values of the nanocomposites also increased with the clay content. It is concluded that partially/intercalated nanocomposites were formed at relatively low clay contents.