F. Calderas

Study of the antioxidant properties of extracts obtained from nopal cactus (Opuntia ficus-indica) cladodes after convective drying

BACKGROUND The process of convective drying was evaluated in terms of the bioactive compounds contained in nopal samples before and after dehydration. Total polyphenol, flavonoid, flavonol, carotene and ascorbic acid contents were determined in undehydrated and dehydrated samples. Two drying temperatures (45 and 65 °C) and two air flow rates (3 and 5 m s(-1) ) were evaluated. The rheology of samples under the best drying conditions was also studied, since it provides important information regarding processing (mixing, flow processing) as well as the sensory attributes (texture) of rehydrated samples. RESULTS Non-Newtonian shear-thinning behaviour was observed for samples dried at 45 °C, while samples dried at 65 °C showed shear-thickening behaviour, possibly caused by thermal chain scission of high-molecular-weight components. CONCLUSION The best conditions for bioactive compound preservation were a drying temperature of 45 °C and an air flow rate of 3 m s(-1) , resulting in 40.97 g phenols, 23.41 g flavonoids, 0.543 g β-carotene and 0.2815 g ascorbic acid kg(-1) sample as shown in table 3.

Influence of maleic anhydride grafting on the rheological properties of polyethylene terephthalate–styrene butadiene blends

Abstract The rheological properties of polyethylene terephthalate (PET)–styrene butadiene rubber (SBR) blends are studied in this work. The shear viscosity was measured in a special capillary device mounted on an injection-molding machine, which provides data on pressure and flow rate. Comparison of the viscosity of the PET–SBR physical blend with that of the blend of PET with maleic anhydride-functionalized SBR (SBRg) provides information on the effect of grafted maleic anhydride chains at the rubber–thermoplastic interface on the rheological properties of the blend. Shear viscosity is a function of the amount of compatibilizer and applied shear stress present in the preparation of the blends. A reduction in the particle size due to increasing screw speed was correlated with a reduction in the shear viscosity and an increase in the impact properties of the blend.

On the yield stress of complex materials

In the present work, the yield stress of complex materials is analyzed and modeled using the Bautista-Manero-Puig (BMP) constitutive equation, consisting of the upper-convected Maxwell equation coupled to a kinetic equation to account for the breakdown and reformation of the fluid structure. BMP model predictions for a complex fluid in different flow situations are analyzed and compared with yield stress predictions of other rheological models, and with experiments on fluids that exhibit yield stresses. It is shown that one of the main features of the BMP model is that it predicts a real yield stress (elastic solid or Hookean behavior) as one of the material parameters, the zero shear-rate fluidity, is zero. In addition, the transition to fluid-like behavior is continuous, as opposed to predictions of more empirical models.

Effect of cholesterol and triglycerides levels on the rheological behavior of human blood

Important public health problems worldwide such as obesity, diabetes, hyperlipidemia and coronary diseases are quite common. These problems arise from numerous factors, such as hyper-caloric diets, sedentary habits and other epigenetic factors. With respect to Mexico, the population reference values of total cholesterol in plasma are around 200 mg/dL. However, a large proportion has higher levels than this reference value. In this work, we analyze the rheological properties of human blood obtained from 20 donors, as a function of cholesterol and triglyceride levels, upon a protocol previously approved by the health authorities. Samples with high and low cholesterol and triglyceride levels were selected and analyzed by simple-continuous and linear-oscillatory shear flow. Rheometric properties were measured and related to the structure and composition of human blood. In addition, rheometric data were modeled by using several constitutive equations: Bautista-Manero-Puig (BMP) and the multimodal Maxwell equations to predict the flow behavior of human blood. Finally, a comparison was made among various models, namely, the BMP, Carreau and Quemada equations for simple shear rate flow. An important relationship was found between cholesterol, triglycerides and the structure of human blood. Results show that blood with high cholesterol levels (400 mg/dL) has flow properties fully different (higher viscosity and a more pseudo-plastic behavior) than blood with lower levels of cholesterol (tendency to Newtonian behavior or viscosity plateau at low shear rates).

Unsaturated polyester-clay slurry nanocomposites

Abstract In this work, nanocomposites were produced with an unsaturated polyester resin and sodium montmorillonite (MMT) slurry. Upon increasing the clay content [from 1 to 5 parts per hundred resin (phr)] increments of 57% and up to 120% were found in the flexural modulus, with respect to that of the resin alone. Using clay slurry, the cross-linking reaction was affected by the clay, in such a way that the gelation temperature and the thermal stability increased. Using transmission and scanning electron microscopy, resin intercalation into the MMT silicate layers was observed and the fracture morphology revealed the effect of the clay slurry on the nanocomposite morphology. Furthermore, shear and oscillatory rheological tests revealed the formation of a structure between resin and clay at low shear rates, and the presence of this structure was evidenced by X-ray photoelectron spectroscopy (XPS) measurements.

VISCOELASTICITY OF ASPHALTS MODIFIED WITH SEBS COPOLYMERS FUNCTIONALIZED WITH VARIOUS AMOUNTS OF MALEIC ANHYDRIDE

Abstract It is known that the microstructure of polymer-modified asphalts (PMA) depends strongly of the characteristics of the polymer. The modifier polymer improves the mechanical properties of pure asphalt when used in paved roads. In this work, asphalt is modified using reactive polymers obtained via functionalization of styrene-ethylene/butylene-styrene (SEBS) copolymers in solution using various amounts of benzoil peroxide (BPO) as initiator and maleic anhydride (MAH). The resulting functionalized copolymer (SEBS-g-MAH) with variable amounts of grafting (variable reactivity) is blended with the asphalt at small contents (2–4 wt. %). The amount of MAH grafting was determined by FTIR, 1HNMR, GPC, DSC and by titration with KOH, and this amount is readily correlated with the improvement of the mechanical properties and thermal stability of the asphalt. In addition, the limit of the polymer-asphalt compatibility is determined for these systems.

Properties of the Entomoparasitic Nematodes (Heterorhabditis bacteriophora) Liquid Culture using a Helicoidal Ribbon Agitator as Rheometric System

The rheological parameters: flow behaviour index n, flow consistency index K and effective viscosity ηe were estimated for the entomoparasitic nematodes Heterorhabditis bacteriophora liquid broth at different culture maturation times. The nematode or nematodes were cultivated during 20 days in a bioreactor, and the growth media inside the bioreactor was enriched with protein and fat sources. Rheological parameters for the heterogeneous suspension were estimated with mixer principles employing a helical ribbon agitator fixed to a rheometer. As the culture matured, n decreased from 0.8 to 0.2 (-) and K increased up to 1200 mPa•sn; ηe showed a non-Newtonian (n<1) behaviour, ηe reached peak values of 0.32 Pa•s for a rotational speed of 0.5 revolutions per second (rps) and 0.048 Pa•s for 2.5 rps. Rheological properties reported here could be more reliable as compared to those reported for non-homogeneous liquid fermentations where estimations were performed with conventional geometries (i.e. concentric cylinders) which are appropriate for homogeneous systems but not for non-homogeneous ones.

Properties of pet-pen blends produced by extrusion and injection blow- molding

In this work, blends made of polyethylene terephthalate (PET) and polyethylene 2,6 naphthalene dicarboxylate (PEN) were prepared by melt extrusion processed at several temperatures and residence times. The blends with composition up to 20 wt % PEN were characterized with regard to their degree of transesterification and their rheological properties. Subsequently, bottles were produced by injection-stretch blow-molding and their mechanical properties were measured. It was found that improved PET-PEN blends were produced at 270°C with a PET content of 15 and 20 wt % and residence time processing of 4 minutes. Both blends present a low degree of transesterification, block conformations, high shear viscosity and improved tensile properties.

Instrumentation and start up of a new elongational rheometer with a preshearing history

This article describes the design, start up, and some preliminary results of a rheometer that preshears the test fluid and measures its elongational viscosities. Preshearing takes place within a concentric cylinders arrangement. Fluid goes out from the lower part of the cylinders arrangement where an elongational flow is produced, by means of a vacuum chamber, which draws out the fluid filament. The influence of the preshearing history upon the elongational properties is analyzed. Data of elongational stress, uniaxial stretch rate, elongational normal stress difference, and apparent elongational viscosity are obtained. The rheometer design is based on a previous work by Sridhar and Gupta [Rheological Act 24, 207 (1985)]. Glycerin was used as the test fluid. Stretch rates up to 1250 s−1 were found. Troutons ratios ranging from 2.6 to 3.6 were obtained.

On the pulsating flow behavior of a biological fluid: human blood

In this work, the rectilinear flow of a complex fluid (human blood) under a pulsating time-dependent pressure gradient is analyzed. A first approximation of the real case of blood flowing in a vein is described. The normalized pressure gradient simulates the pumping work of the heart while the flow geometry (circular tube) is assumed rigid, smooth, and cylindrical. The rheological behavior of blood with different cholesterol levels is modeled using the Bautista–Manero–Puig (BMP) constitutive equation. According to the analytical solution, a flow enhancement is predicted to first order which represents the optimum pumping work of the heart which governs the flow of blood in the entire body. This work is a contribution to the understanding of the complex rheology involved in the discontinuous pressure-driven flow of blood in the human body.