Impregnation Stage Research Articles

Fabrication of high aluminium containing rare-earth doped fiber without core–clad interface defects

The unwanted core–clad interface defect generation and related scattering loss in rare-earth doped optical fibers containing Al-oxide is up to now an unsolved problem as it cannot be avoided without modifying fiber design or composition. Here a simple technique is proposed to eliminate the problem without affecting fiber parameters. The method comprises addition of fumed silica in appropriate amount under dispersed condition to the soaking solution containing Al and rare-earth salts during the solution impregnation stage. This facilitates the networking between Al 2O 3 and SiO 2 during sintering. The method leads to an improvement in optical properties of the fibers and enables doping of high concentration of Al-oxide even in presence of GeO 2 in the core.

Influence of preparation conditions on porous structures of olive stone activated by H3PO4

An olive factory residue was used as a precursor in the preparation of granular activated carbon by chemical activation with H3PO4. Effects of final activation temperature, time, and H3PO4 concentration used in the impregnation stage on the porous development were investigated. SO2 adsorption experiments were also performed for some of the activated carbon samples to represent their adsorption performance. Activation at low temperature represented that micropores were developed first at early stages of the temperatures. Mesoporosity developed at around 250 °C, enhanced up to 400 °C, and then started to decrease due to possibly shrinking of pores. The optimum temperature for olive stone was found to be around 400 °C on the basis of total pore volume and BET surface area. It was clearly demonstrated that H3PO4 concentration used in the impregnation stage was not only effective for development of surface area and pore volumes but also an effective tool for tailoring the pore structure and size distribution.

Permeability prediction for the meso–macro coupling in the simulation of the impregnation stage of Resin Transfer Moulding

The impregnation stage of the Resin Transfer Moulding process can be simulated by solving the Darcy equations on a mould model, with a ‘macro-scale’ finite element method. For every element, a local ‘meso-scale’ permeability must be determined, taking into account the local deformation of the textile reinforcement. This paper demonstrates that the meso-scale permeability can be computed efficiently and accurately by using meso-scale simulation tools. We discuss the speed and accuracy requirements dictated by the macro-scale simulations. We show that these requirements can be achieved for two meso-scale simulators, coupled with a geometrical textile reinforcement modeller. The first solver is based on a finite difference discretisation of the Stokes equations, the second uses an approximate model, based on a 2D simulation of the flow.

Refining Energy Reduction and Pulp Characteristic Modification of Alkaline Peroxide Mechanical Pulp (APMP) Through Enzyme Application

Eucalyptus wood chips were subjected to impregnation with various blends of novel fiber modify-ing enzymes before chemical pretreatment and two stages of refining using the preconditioning refiner chemical–alkaline peroxide mechanical pulping (PRC-APMP) process. Wood chip impregnation and pulp processing was con-ducted at a pilot plant in the United States. When compared under constant chemical application and at a constant 350 mL CSF, enzyme treatment reduced specific refining energy by at least 24%. The effect of one versus two stages of impregnation and of enzyme action upon several physical pulp properties was determined.

Preparation and Characterization of Expandable St/MMA Copolymers Produced by Suspension Polymerization

Free radical suspension copolymerization of styrene (St) with methyl methacrylate (MMA) in the presence of n-pentane was investigated. The batch polymerization was performed in a stirred reactor via a two-stage process. First, free radical suspension polymerization of St and MMA in aqueous media was carried out at about 80—90°C with the aid of a monofunctional initiator to the monomer conversions up to about 70%. Second process, called impregnation stage, consisted of a high temperature—high pressure cycle (110—120°C and 10 bars) in which the blowing agent was charged in the reactor and polymerization was carried out with the aid of a high temperature initiator. Fine particles of micron sizes of St/MMA copolymer were synthesized by manipulation of some experimental parameters governing the copolymerization system. Polymerization was carried out at different agitation rates, suspending agent and aqueous phase initiator concentrations as well as emulsifier amounts, while keeping constant the MMA/St ratio. To reach a desired bead size, the above parameters were optimized using the Taguchi method for experimental design and the relative importance of the mentioned parameters was analyzed. Particles with a spherical shape of 250—350 μm were formed. The obtained copolymers were also characterized in terms of molecular weight, polydispersity index, copolymer composition, morphology of the polymer beads, thermal behavior, and particle size and particle size distribution.

Effects of styrene oligomers and polymers on the suspension polymerization behavior and properties of expandable polystyrene

AbstractStyrene oligomers are formed by a free‐radical mechanism during the thermal polymerization of styrene in storage. The effects of these compounds on the preparation of expandable polystyrene (EPS) were investigated with respect to suspension polymerization behavior and the properties of the impregnated polystyrene beads produced. Styrene dimers and trimers up to concentrations of 0.2 wt % did not affect the stability of the suspension during the polymerization and impregnation stages. Besides differentiated effects on the particle size distributions of the polymers and on the polymerization rate, no chain‐transfer activity of the oligomers was observed, and this confirmed the assignment of chain transfer to the Diels–Alder dimer in the literature. The investigation of the foaming behavior of the pentane‐impregnated EPS beads indicated a significant reduction of the prefoaming density caused by styrene dimers and trimers. This behavior resulted from a decrease in the glass‐transition temperatures of these polymers. The effects of high‐molecular‐weight polystyrene, formed in addition to oligomers during storage by the thermal polymerization of styrene, on the polymerization behavior and polymer properties of EPS were also investigated. The results showed a significant impact on the suspension stability that was dependent on the concentration of the high‐molecular‐weight polystyrene. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Permeability of textile reinforcements: Simulation, influence of shear and validation

The permeability of textile reinforcements is a crucial input for the simulation of the impregnation stage of a composite material fabrication process. In this paper, we present a fast and accurate simulation method for the permeability of a textile reinforcement, based on a finite difference discretisation of the Stokes equations. Results for single layer, multi-layer and sheared models are discussed. The influence of intra-yarn flow and periodic or wall boundary conditions are considered. We compare the numerically computed permeability values with experimental data.

Production of supported asymmetric catalysts in a fluidised bed

The production of supported catalysts by impregnation method of porous supports can be carried out in a fluidized bed. This technique allows to obtain, in only one apparatus, catalysts which by the traditional way must successively undergo the stages of impregnation, filtration, drying and calcination/activation. This paper deals with the results of experiments related to the manufacture of traditional catalysts and new generation catalysts containing various metals (nickel, manganese, copper, palladium) deposited on a porous support such as alumina or silica gel. Moreover, it was shown that the reproducible synthesis of metallic nanoparticles of size, composition and controlled surface quality could be carried out under mild conditions with organometallic precursors. The principle of this technique consists in the spraying of a solution containing a metallic precursor into a hot fluidised bed of porous fine particles as chosen support. The adequate choice of operating conditions makes possible a uniform deposit of the metal precursor inside the porous matrix. After the impregnation step, the decomposition of the metallic precursor and metal activation can be operated in the same reactor. For organometallic complexes the decomposition/activation is achieved at low temperature ( T < 80 °C) under a reactive atmosphere (H 2/N 2 mixture) giving then rise to the synthesis of metal nanoparticles with controlled size and composition. Feasibility studies and optimisation of the operating conditions have been carried out, solid characterization and catalytic tests have been conducted to validate the process. In the case of asymmetric catalysts, Pd nanoparticles associated to cinchonidine as an asymmetric ligand inside silica particles have been prepared by this process for the ethyl pyruvate hydrogenation.

Injection molding of long sisal fiber-reinforced polypropylene: Effects of compatibilizer concentration and viscosity on fiber adhesion and thermal degradation

A two-step process was used to obtain long sisal fiber-polypropylene (SF/PP)–reinforced thermoplastic composites, using maleic anhydride grafted polypropylene (MA-g-PP) as a compatibilizer. At a first stage, modified polypropylenes (mPP) were used for an extrusion impregnation process, for the preparation of composite pellets containing about 70 wt% of SF. SF/mPP pellets with a large aspect ratio were prepared by continuous extrusion impregnation of a continuous SF yarn, using a single screw extruder and an adequate impregnation die. The mPP used were MA-g-PP and regular polypropylene (PP), modified by reaction with different amounts of an organic peroxide. The composite pellets were thus dry blended with regular PP pellets in an injection machine hopper, and injection molded to obtain composite tensile specimens with a minimum quantity of modified polypropylene, minimum fiber breakage and thermal degradation, and excellent mechanical properties. It is shown that the fiber breakage is reduced to a minimum, even for recycled composites, due to the presence of the low-viscosity polymer layer wetting the SF fibers. The bulk composite effective viscosity and the fiber breakage extent and thermal degradation during the injection-molding step are found to be closely related. Blending with much less expensive mPP at the impregnation stage optimizes the amount of expensive MA-g-PP. POLYM. ENG. SCI., 45:613–621, 2005. © 2005 Society of Plastics Engineers

Numerical model of relaxation during vacuum-assisted resin transfer molding (VARTM)

The inherent economic and environmental advantages of a vacuum-assisted resin transfer molding (VARTM) composite manufacturing process have caused an increase in the number of applications and generated research activity within the industry. Unfortunately, a variation in part thickness is introduced during the impregnation stage of a vacuum-infused flexible mold process, leading to a predictable and undesirable reduction in mechanical properties. It will be shown that the fluid pressure gradient established during the infusion phase directly results in a gradient in part thickness. The severity of the thickness gradient depends on the magnitude of the imposed pressure and the behavior of the underlying fibrous material. This report examines this behavior through the development of an efficient numerical model. An equilibrium relationship between external confining pressure, reinforcement response, and internal resin pressure is proposed and implemented within the context of a specific implicit finite element-based procedure. A detailed description of the original algorithm provides the background for the insertion of the new equilibrium constraint. This report also includes a discussion of important issues such as information recovery and adaptive time-step procedures. The result is a simple numerical tool capable of simulating the infusion process. The new procedure is validated through a comparison with known analytic one-dimensional models and also used to obtain solutions to an important class of axisymmetric examples. POLYM. COMPOS., 26:628–635, 2005. © 2005 Society of Plastics Engineers

Free boundary viscous flows at micro and mesolevel during liquid composites moulding process

AbstractNumerical simulation aspects, related to low Reynolds number free boundary viscous flows at micro and mesolevel during the resin impregnation stage of the liquid composite moulding process (LCM), are presented in this article. A free boundary program (FBP), developed by the authors, is used to track the movement of the resin front accurately by accounting for the surface tension effects at the boundary. Issues related to the global and local mass conservation (GMC and LMC) are identified and discussed. Unsuitable conditions for LMC and consequently GMC are uncovered at low capillary numbers, and hence a strategy for the numerical simulation of such flows is suggested.FBP encompasses a set of subroutines that are linked to modules in ANSYS. FBP can capture the void formation dynamics based on the analysis developed. We present resin impregnation dynamics in two dimensions. Extension to three dimensions is a subject for further research. Several examples are shown and efficiency of different stabilization techniques are compared. Copyright © 2004 John Wiley &amp; Sons, Ltd.

On-Line Characterization of Bulk Permeability and Race-Tracking During the Filling Stage in Resin Transfer Molding Process

Resin Transfer Molding (RTM) is widely used to manufacture polymer composite materials. In this process, the fiber preform is placed in a closed mold and thermoset resin is injected to saturate the preform. After the resin cures the mold is opened and the net shape composite part is obtained. With RTM, one is capable of making complex and high quality composite parts with short cycle times. However, by introducing more complexity into the part, one also introduces higher probability of disturbances, such as race-tracking of resin during impregnation along preform edges. This can lead to incomplete saturation of fiber preform forming flaws such as dry spots in the composite part. The strength and existence of race-tracking is a function of the fabric type, perform manufacturing method, and their placement in the mold. It can vary from one part to the next in the same production run and usually it is not repeatable. The characterization of race-tracking and preform permeability is a key input for simulation tools that optimize the gates and vents locations and for designing advanced sensing and flow control strategies. This paper describes a method to estimate the strength and location of race-tracking and characterize the preform bulk permeability during the resin impregnation stage of RTM. The approach can be described in two steps. The first step generates simulations off line that map the overall range of potential disturbances likely to occur during the injection. The second step uses sensors placed in the mold, which allow one to identify the simulation within the database that resembles the experiment in terms of the filling behavior during the filling stage on the manufacturing platform. In the final step, this information is used to estimate race-tracking strength and location as well as the bulk permeability. Both experimental and numerical case studies show the reliability and accuracy of this method. It could be utilized for a broad range of applications such as active flow control, vent location optimization, and process monitoring of mold filling processes.

Numerical model for vacuum infusion manufacturing of polymer composites

The focus is set on the development and evaluation of a numerical mgodel describing the impregnation stage of a method to manufacture fibre reinforced polymer composites, namely the vacuum infusion process. Examples of items made with this process are hulls to sailing yachts and containers for the transportation industry. The impregnation is characterised by a full 3D flow in a porous medium having an anisotropic, spatial‐ and time‐dependent permeability. The numerical model has been implemented in a general and commercial computational fluid dynamic software through custom written subroutines that: couple the flow equations to the equations describing the stiffness of the fibre reinforcement; modify the momentum equations to account for the porous medium flow; remesh the computational domain in each time step to account for the deformation by pressure change. The verification of the code showed excellent agreement with analytical solutions and very good agreement with experiments. The numerical model can easily be extended to more complex geometry and to other constitutive equations for the permeability and the compressibility of the reinforcement.

Interactions between polysulphides and stainless steel materials

AbstractAbility of polysulphide to enhance the potential of digester materials (S30400, S31603, S32205, S32304) was studied in the impregnation stage of the pulping process. Polysulphides were generated by adding elemental sulphur to alkaline sulphide solutions. Polysulphide levels 0.3–0.48, 1.25–2, 5–8 and 12.5–20 g/l enhanced the potentials of the steels by 70, 90, 170 and 190 mV, respectively. The polysulphide concentrations were calculated values, because concentrations of polysulphides are very difficult to measure in kraft liquors. No weight changes were observed, but slight accumulation of sulphur onto surfaces, particularly onto S31603 was seen in X‐ray fluorescence analysis. Otherwise, differences in behaviour among the steel materials were minimal.

Diffusion and Reaction in Isothermal Pulping Digesters

A mathematical model describing isothermal alkali diffusion and reaction with acetyl groups in moist wood chips was derived and validated against experiments. A coupled mass balance for the acetyl groups and the alkali was posed and solved numerically. Differences observed earlier between model-predicted and experimentally obtained alkali concentration profiles demonstrated that the diffusion coefficient D is not a constant but a certain function of alkali concentration C. A model for this dependence was developed on the basis of theoretical principles. The function C→D plays a key role in the qualitative form of the resulting alkali profiles, especially in terms of changes in the sign of the curvature that were not explained by other approaches in the literature. A new parameter, the “deacetylation index”, was introduced as an observable quantity that is useful for tracking the end of the impregnation stage in pulping processes.

Sulfidation and Thiophene Hydrodesulfurization Activity of Nickel Tungsten Sulfide Model Catalysts, Prepared without and with Chelating Agents

Silica-supported NiWS catalysts with a high activity for thiophene hydrodesulfurization are obtained when chelating agents such as 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA), ethylenediaminetetraacetic acid, or nitrilotriacetic acid are added in the impregnation stage. X-ray photoelectron spectroscopy has been used to follow the state of Ni and W during the temperature programmed sulfidation of Ni–W model catalysts prepared with and without chelating agents on planar SiO2 films on silicon substrates. Fully sulfided catalysts have been tested in thiophene hydrodesulfurization. The activity increases with increasing Ni content and reaches a plateau at a Ni : W atomic ratio of 0.66. In NiW catalysts prepared without additives the sulfidation of Ni precedes that of W. However, Ni sulfide formed at low temperatures changes its structure at high temperatures where WS2 is present, as indicated by the Ni XPS binding energy, which we tentatively attribute to redispersion of sulfidic Ni over WS2. Chelating agents stabilize Ni against sulfide formation at low temperature, the effect being strongest when CyDTA is applied. CyDTA retards the sulfidation of Ni to temperatures where all W has already been sulfided. This complete reversal in the order in which the two elements convert to sulfides is seen as the key step in preparing highly active NiWS HDS catalysts.

Impregnation of Fibrous Materials with Lc Polyesters and Blends on Their Base

Abstract Impregnation and infiltration of fibrous materials by LC copolyester melts and the blends with viscous thermo-plastic polysulphone were quantitatively described as unsteady and steady stages in terms of rheological characteristics of the melts using an equivalent capillary model. Addition of LC polyester to viscous thermoplastic melt noticeably enhances impregnation condition. Study of the melt flow through various capillaries demonstrated a development of the melt slippage at high shear rate along the capillary walls. Slippage phenomena may be important at the initial stage of impregnation but can be neglected for final stage due to low shear rate in the last case. On the other hand, at those conditions a yield stress of the melt may considerably complicate the impregnation process due to a drastic viscosity increase.

The structure-texture relationship of blanched and glucose-impregnated strawberries / Relación entre la textura y la estructura de fresas escaldadas e impregnadas de glucosa

The effect of water or steam blanching, glucose impregnation to attain an equilibrium water activity ( aw) value of 0.93 or 0.95, and the addition of calcium on textural characteristics of strawberries was analysed. Strawberries were softened by blanching, varying the maximum extrusion force from 53 to 82% of the value of the fresh fruit for water or vapour treatment, respectively. Major changes in texture occurred during blanching, while the influence of the impregnation step depended on the previous treatment. Addition of 0.1% (w/w) calcium lactate during the impreg nation stage improved the textural characteristics of vapour-blanched glucose-treated fruits but was ineffective in preventing softening of water-blanched glucose-treated strawberries. Ultrastructural changes, studied by electron microscopy, correlated well with changes in texture.

Investigations into various methods of liquid injection to achieve mouldings with minimum void contents and full wet out

Much potential and interest exists for processing composite structures using liquid injection due to the lower cost possibilities when compared to that for wet band lay-up autoclave techniques. The specification, however, for high-performance structures requires low void contents below 1% and high fibre volume fractions in excess of 50%, both of which are currently possible with existing autoclave technology. This investigation reports on various methods for resin transfer and gating to maximize fabric wet out and minimize void content. The initial tests were conducted using a volume fraction of 18% for plain weave glass fibre to identify any variations in processing through visual quality and mechanical properties. Composite plaques of 50% fibre volume were then manufactured using radial and peripheral gating from which the differences in wet out, void content, and process injection times are reported. This investigation concludes that peripheral gating using vacuum impregnation produced the optimum composite structure in terms of void content and visual transparency. This method of gating also achieved the lowest injection time during the impregnation stage of processing for fibre volumes of 18%. Radial gating failed to completely fill the reinforcement at 50% fibre volume using the same thermal and pressure conditions as peripheral gating. Thickness variation studies also revealed inadequate mould tool stiffness when subjected to pressure during resin injection.

Neoformation of DLH During Impregnation of α-Alumin

The adsorption of Co(II) or Ni(II) ammine complexes from aqueous solutions onto γ-alumina at neutral pH and ambient temperature was investigated. The formation of coprecipitates including Al (III) ions extracted from the support was demonstrated by EXAFS for contact times and Ni or Co loadings higher than 0.5 h and about 2.0 wt %, respectively. The EXAFS technique makes it possible to distinguish the Ni or Co hydroxides and basic nitrates from coprecipitates with a double layer hydroxide (DLH) structure. Not only is EXAFS shown to be sensitive to the presence of aluminum in the coprecipitates, but in most cases, the M(II)/Al(III) ratio (M= Ni or Co) in the supported coprecipitates can be estimated. Thus, alumina should not be considered systematically as inert even at pH values close to its isoelectric point. It is suggested that a dissolution-precipitation mechanism is involved and that the rate of alumina dissolution is promoted by adsorbed Ni(II) or Co(II) ions. Site-binding models have a considerable value for the early stages of impregnation, whereas a geochemical approach involving surface rehydration and coprecipitation have probably a greater validity for the later stages.