Halil Gevgilili

Wall slip and extrudate distortion of three polymer melts

A high-density polyethylene (HDPE), a poly(dimethylsiloxane) (PDMS), and an oxetane based alternating block thermoplastic elastomer (TPE) were subjected to steady torsional and capillary flows. HDPE and PDMS exhibited wall slip in steady torsional flow. The shear stress ranges at which wall slip became apparent during steady torsional flow for HDPE and PDMS coincided with the wall shear stress ranges in capillary flow at which distortions of the extrudates emerging from capillary die were first noted. In contrast, TPE did not exhibit wall slip in steady torsional flow (for shear rates up to 200 s−1 and strains up to 25). None of the three polymers exhibited any overshoots of shear stress or first normal stress difference under steady flow conditions, indicating that the often-reported stress overshoots could be artifacts of wall slip. Furthermore, the extrudates of the TPE were relatively smooth and exhibited only minor and nonperiodic surface blemishes. The absence of extrudate distortions under the pres...

Step strain flow: Wall slip effects and other error sources

The traditional technique for the experimental characterization of the shear stress relaxation modulus by applying a step shear strain was investigated using flow visualization. A high-speed camera was employed in conjunction with cone-and-plate and parallel-disk fixtures of a Rheometric Scientific Advanced Rheometric Expansion System rheometer. In the nonlinear region the true shear strain imposed on a polyethylene melt deviates considerably from the targeted strain. The main source of the deviation is the wall slip of the polymer melt. The presence of wall slip reduces significantly the range of strains for which the strain-dependent relaxation modulus can be determined for the linear polyethylene melt. Errors associated with the control of the motion of the tool which introduces the shear strain are also documented.

Biomass pretreatment strategies via control of rheological behavior of biomass suspensions and reactive twin screw extrusion processing

Twin screw extrusion based pretreatment of biomass is an attractive option due to its flexibility to carry out chemical reactions under relatively high stresses, temperatures and pressures. However, extrusion processes are rarely utilized in biomass pretreatment because such processing is constrained by rheological behavior of typical biomass suspensions. Without the manipulation of their rheological behavior, biomass suspensions become unprocessable within the extruder at modest biomass concentrations. Here it is demonstrated that gelation agents can render biomass suspensions processable. Specifically, carboxy methyl cellulose, CMC, could be used in conjunction with alkaline pretreatment of hardwood-type biomass and enabled separation of lignin from cellulose fibers. Furthermore, recycled black liquor, obtained upon pretreatment, was determined to be as effective as CMC for rendering biomass suspensions flowable by again facilitating the concomitant application of high shearing stresses and chemical treatment for the pretreatment of the biomass in the twin screw extruder.

Shell-core bi-layered scaffolds for engineering of vascularized osteon-like structures.

Bottom-up assembly of osteon-like structures into large tissue constructs represents a promising and practical strategy toward the formation of hierarchical cortical bone. Here, a unique two-step approach, i.e., the combination of electrospinning and twin screw extrusion (TSE) techniques was used to fabricate a microfilament/nanofiber shell-core scaffold that could precisely control the spatial distribution of different types of cells to form vascularized osteon-like structures. The scaffold contained a helical outer shell consisting of porous microfilament coils of polycaprolactone (PCL) and biphasic calcium phosphates (BCP) that wound around a hollow electrospun PCL nanofibrous tube (the core). The porous helical shell supported the formation of bone-like tissues, while the luminal surface of nanofibrous core enabled endothelialization to mimic the function of Haversian canal. Culture of mouse pre-osteoblasts (POBs, MC 3T3-E1) onto the coil shells revealed that coils with pitch sizes greater than 135 μm, in the presence of BCP, favored the proliferation and osteogenic differentiation of POBs. The luminal surface of PCL nanofibrous core supported the adhesion and spreading of mouse endothelial cells (ECs, MS-1) to form a continuous endothelial lining with the function similar to blood vessels. Taken together, the shell-core bi-layered scaffolds with porous, coil-like shell and nanofibrous tubular cores represent a new scaffolding technology base for the creation of osteon analogs.

Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and asphalt

The effectiveness of the treatment of chromite ore processing residue (COPR) with ferrous sulfate and encapsulation into asphalt were explored separately and in combination. The asphalt treatment was conducted by mixing COPR or ferrous sulfate pretreated COPR with varying amounts of asphalt. To assess the efficacy of the treatment, the leachability of toxicity characteristic leaching procedure (TCLP) total chromium (Cr) from all treated samples was determined for curing periods up to 16 months. X-ray absorption near edge structure (XANES) analyses were also performed to evaluate the Cr(6+) concentration in the selected samples. The combination treatment of ferrous sulfate and the encapsulation of the treated COPR into asphalt reduced the TCLP total Cr concentration to lower than the regulatory limit of 5mg/L for Cr contaminated soils, after 16 months. However, the Cr concentrations were still higher than the universal treatment standards (UTS) of 0.6 mg/L for hazardous waste. On the other hand, treatment with ferrous sulfate alone or the encapsulation of the COPR in asphalt failed to meet the TCLP total Cr concentration of 5mg/L, after 16 months. XANES analyses results showed that more than 75% Cr(6+) reduction was achieved upon pretreatment with ferrous sulfate.

Use of Adjustable-Gap On-Line and Off-Line Slit Rheometers for the Characterization of the Wall Slip and Shear Viscosity Behavior of Energetic Formulations

It is difficult to characterize the rheological behavior of energetic suspensions due to their viscoplasticity and wall slip. The use of the rectangular slit geometry, as an on-line or off-line rheometer is advantageous provided that the surface to volume ratio of the slit die can be systematically varied to allow the wall slip corrections to be made. Here two rectangular slit rheometers designed and built to handle the rheological behavior of energetic suspensions are presented. The gap of these rheometers is variable to give the user the ability to vary the gap and hence the wall shear rate, thus enabling wall slip corrections to be made. A series of pressure transducers, flush with the wall, are used to determine the pressure drop over the fully developed flow region and hence the wall shear stress directly from the fully developed pressure gradient. The ability to independently vary the mass flow rate and the gap opening allows one to carry-out the wall slip corrections and hence obtain the wall slip velocity versus the shear stress data, which can then be used as the boundary condition during the simulation of the die and extrusion flows and at the same time allow the determination of accurate shear viscosity data. A set of data systematically collected with an on-line slit rheometer with a continuously adjustable gap to characterize the wall slip velocity as well as the shear viscosity material function of a LOVA formulation as a function of deformation rate, solvent concentration, and temperature is used to illustrate the working principles of the on-line and off-line adjustable-gap rheometers.

Effects of dispersion and deformation histories on rheology of semidilute and concentrated suspensions of multiwalled carbon nanotubes

A new sonication method involving in situ measurement of electrical conductivity along with the characterization of dynamic properties was developed to better control the dispersion of multiwalled carbon nanotubes (CNTs) in polymeric liquids. Using this new method, CNTs were incorporated into monoacrylate-terminated poly(ethylene glycol) at CNT volume fractions, ϕ, in the semidilute (3.7 × 10−4 and 5.5 × 10−3) and concentrated (1.1 × 10−3, 1.6 × 10−3 and 2.7 × 10−3) regimes. This dispersion methodology enabled the preparation of stable CNT suspensions, with reproducible electrical conductivity and dynamic properties, to serve as specimens for subsequent steady and oscillatory shear, and compressive squeeze flows that were carried out in combination with optical imaging and continuous monitoring of their electrical conductivity. During small-amplitude oscillatory shear, the networks established by well-dispersed CNTs remained stable, as evidenced by optical imaging and constant values of electrical conduct...

Rheological Characterization of Nitrocellulose Gels

Various nitrocellulose, NC, based propellant formulations need to be processed using new and more environmentally friendly solvent combinations on the one hand and using continuous processing methodologies on the other hand. A detailed understanding of the significant changes that take place in the structure and hence the rheological behavior of NC based formulations during manufacture is required to minimize the use of organic solvents and to revert to safer and green solvents. Towards achieving these objectives, experimental methodologies were developed for the first time to enable the accurate characterization and thus fingerprinting of the rheological behavior of NC gels. In these methodologies linear viscoelastic measurements are employed. The concentration of the solvents existing in the gel sample during rheological characterization is concomitantly monitored to allow the documentation of the major source of error associated with the rapid loss of the typical solvents, which generally exhibit relatively high vapor pressures. These measurements have indicated that the source of the NC fibers and the treatment method alter the rheological behavior and can be tracked. The rheological properties of the NC gels can be linked to their manufacturability and such data can be used to pinpoint optimum geometries and processing conditions. The processability of the NC based formulations can also be tailored on the basis of rheological characterization, allowing the manufacturers greater latitude for reducing costs and environmental footprint during manufacture, as well as improving the quality of their NC based energetic formulations.

Twin-Screw Extrusion of Nano-Alumina–Based Simulants of Energetic Formulations Involving Gel-Based Binders

A 7.5-mm twin-screw extruder was developed specifically for the processing of energetic formulations involving nanoparticles. Prior to extrusion of energetic formulations, simulants of CMC, water, and alumina nanoparticle gels were extruded. Quantitative measures of degree of mixedness (statistics of concentration distributions) were obtained on samples processed with the twin-screw extruder and with conventional processing methods using wide-angle X-ray diffraction (WAXD) and thermo-gravimetric analysis (TGA) and were corroborated with microscopy. Twin-screw extrusion process generated more homogeneous mixtures of nanoparticles than conventional (intensive batch) mixing technologies and the use of surfactants further improved the homogeneity. With increasing homogeneity the suspension exhibited lower elasticity and shear viscosity. Overall, the results of this study emphasize the important roles played by the surface properties of rigid particles, the interfacial tension between the particles and the binder, and the rheological behavior of the binder. In the absence of properly selected binder and surfactant(s), the processing of nanoparticles, without agglomeration, is difficult to achieve. This finding may be relevant to the evaluation of past efforts, which have aimed to improve the ultimate properties of energetic formulations by incorporating nanoparticles.

Safety in Design and Manufacturing of Extruders Used for the Continuous Processing of Energetic Formulations

Extruders used for the continuous processing of energetic materials require various types of safety features and thus are differentiated from the extruders commonly available to civilian industries. Items of particular importance to the user include the in-process volume, control of the energetic material properties (especially temperature and pressure), the ability to quickly release pressure, reduction of metal-to-metal contact, and the control of electrical discharge. In this article, two novel extrusion platforms, the first one involving a flexible manufacturing platform and the second designed to process nanoenergetics, are described to illustrate the procedures necessary to design extrusion platforms for energetics manufacture. Particular emphasis is given to the safety features that need to be incorporated during the design stage, along with a detailed discussion of the flexibility and ease of use of extrusion equipment. The use of material-specific mathematical modeling in the design of the extrusion platforms is also elucidated as a first line of defense for safety and ease of use.