No abstract

No abstract

ABSTRACT:The pressure drop resulting from polyamide 6 flow through industrial spinnerets and wire-mesh filters was examined as a possible parameter for improving spinning process constancy with experimental techniques and a numerical approach. The rheological characterization of the polymer melt was performed with a capillary rheometer and a controlled-stress rotational rheometer equipped with a high-temperature oven cell. Measurements in a nitrogen atmosphere were carried out at different temperatures and at various moisture contents to determine the effect of the postcondensation process on the rheological properties of the polymer melt. These experiments were used to collect all basic material information necessary to fit the data with the purely viscous Cross model and the viscoelastic Kaye, Bernstein, Kearsley, Zappas (K-BKZ) model. A spinning pilot plant (consisting of an extruder, a gear pump, a pressure sensor, and a spin beam with several spin packs installed) was used to measure pressure drop values through industrial spinnerets and through two types of filters: (1) Dutch twilled weave filters and (2) sintered filters. Pilot plant tests on filters showed that in the examined range of melt throughputs, the pressure drop increased linearly with an increase in the melt flow rate for all the filters considered. The results with respect to the spinneret geometry led to the conclusion that the numerical simulations gave satisfactory predictions even for experimental data coming from complex systems such as spinning plants, as long as extensional properties were accounted for by the model. On the contrary, pressure drop predictions obtained from the Cross model underestimated the pilot plant values by approximately 20% because of the inability of the model to consider the extensional component of the flow.

No abstract

The effects of concentration of polyacrylic acid as a dispersant on rheological properties of aqueous alumina suspensions have been investigated under steady and oscillatory shear conditions. At solid volume fractions between 0.45 and 0.6, a high degree of particle stabilization is achieved when 0.2 wt% of polyacrylic acid is added. At lower dispersant concentrations, suspensions exhibit pronounced irreversible thixotropic behaviour, whereas at higher dispersant concentrations, time dependent effects on the flow properties are not detectable. When the saturation adsorption limit of the polyelectrolyte on AI,O, is reached, further addition of the dispersant appreciably changes the flow behaviour, as well as the viscoelastic response of investigated suspensions. The data under steady shear are described by application of the generalized Casson model, and for the analysis of viscoelastic data the generalized Maxwell model is used. Keywords: a alumina suspensions, irreversible thixotropy, yield stress, viscoelastic properties, particle stabilization.ighly concentrated aqueous suspensions are very H important raw materials in advanced ceramic processing, such as slip casting, injection molding, etc. The formulation and preparation of highly concentrated suspensions implies rational criteria in order to achieve a high degree of particle stabilization resulting in relatively low viscosity and adequate viscoelastic properties. When stable highly concentrated alumina suspensions are to be prepared in aqueous disperse media, it is necessary to select the type of particle stabilization (based on electrostatic, steric or electrosteric repulsion) and to find the additive amount necessary to reach the optimal conditions. In several industrial applications of highly concentrated ceramic suspensions, particle stabilization by addition of polyelectrolytes appears to be favourable in order to overcome the problems connected with ageing and processing of multiphase systems.Ammonium and sodium salts of polyacrylic (PAA) and polymethacrylic (PMAA) acids are widely used in the ceramic industry as dispersants for aqueous suspensions. The functional groups of these polymers are carboxylic groups, which can be COOH or COO-, depending on the pH of the solution. Cesarano I11 et al. (1988) studied the influence of PAA and PMAA on the stability of aqueous alumina suspensions. They showed that at pH values exceeding the zero point of charge (zpc) on the surface of the alumina particles, these polymers are hlly dissociated and polyelectrolyte adsorption in this pH range is still controlled by the minority *Author to whom correspondence may be addressed. E-mail address: Andreja.Valant@,uni-lj.si of positive surface sites, even though the net surface charge is negative. When pH increases, the amount of adsorbed polymer necessary to provide stabilization decreases. Above pH 9, high-affinity-type polymer adsorption does not occur and the presence of excess polymer in solution is inevitable. In order to reach the saturation adsorption limit there ...