Kinetics of convective drying of fibre materials based on solution of a diffusion equation

The calculated kinetic curves of convective drying for the moisture, rate, and acceleration of drying are proposed based on the solution of a diffusion equation. The results of the calculation with the equations obtained are compared with the experimental data for convective drying of fibre materials: cotton and viscose fibre and cotton cloth. The universality of the character of the equation of the dimensionless rate of drying as a function of criterion E (dimensionless time of the process) was demonstrated, and this allows generalizing the experimental data on drying of fibre materials. Translated fromKhimicheskie Volokna, No. 6, pp. 53–56,November–December, 1998.     

Carrier films behavior during thermoforming process studied using dynamic mechanical thermal analysis (DMTA)

Dynamic mechanical thermal analysis was used to investigate the thermal transitions and modulus/temperature behavior of thermoformable carrier films, and to relate the information obtained to carrier film behavior during the thermoforming process. In this study the glass transition temperatures (T _g) and the temperatures at which crystallization occurred during heating (T _c) of four thermoformable carrier films were measured by using a dynamic mechanical thermal analyzer (DMTA). These films are good candidates for the automotive process, which uses painted carrier films as moldable automotive coatings (MAC). The modulus/temperature behavior of the films was also observed over a wide temperature range, which included thermoforming temperatures. Although films of PETG and PCTG 5445, co-polyesters based on poly(1,4-cyclohexylene dimethylene terephthalate), are thermoformable, their T _g values, 92 and 99 °C, respectively, are not high enough to allow current paint systems (with bake temperature of 100–110 °C) to cure on the films without causing severe film deformation.     

Sensitivity of operating conditions and material properties for thermoforming process

Abstract

The thermoforming process involves three stages: sheet reheat; forming; and solidification. A polymeric sheet is heated in an oven to the desired forming temperature distribution. The sheet is then deformed to take the shape of the mould cavity and subsequently solidified. The deformation of the sheet is assisted by the application of a pressure differential and/or the use of a moving plug.     

Instrumented thermoforming of advanced thermoplastic composites. II: Dynamics of double curvature part formation and structure development from PEEK/carbon fiber prepreg tapes

Poly(ether ether ketone) (PEEK) carbon fiber prepreg tapes (APC-2) have been thermoformed into a hemispherical double curvature part under a variety of processing conditions. Conventional matched die molding using aluminum molds (at 200°C) were not successful in thermoforming acceptable parts. Parts with severe wrinkling and folding were obtained. A novel three-piece (steel) mold with built-in sheet clamping arrangement was, therefore, designed and fabricated. This mold was used at 400°C temperature to thermoform parts from preheated preconsolidated laminates. More interestingly, using the above conditions, 8- and 16-ply unconsolidated laminates could be directly thermoformed into parts that were microstructurally sound and exhibited good shape conformity. Results suggest a cycle time of 15 min, with scope for further reduction, if mold cooling is employed. Notwithstanding the simplicity of the thermoforming process, such a short cycle time compares quite favorably with cycle times of several hours for conventional thermosetting resin based composites.     

Sag in thermoforming

In thermoforming, flat sheets of plastics are transformed into complex shapes by first softening the sheet, and then by shaping it. While softening, the sheet can extend under its own weight, making it concave. Called sag, this affects how each element of the sheet surface views the heater bank. This can worsen the variations in the heat flux distribution across the sheet. It can also cause the heat flux over and under each element on the sheet to differ. The resulting temperature gradients, either across the sheet or through its thickness, can complicate processability. To suppress sag, practitioners stretch the sheet laterally, using cambered transfer rails to keep the sheet taut. In this article, we model sag analytically, using transport phenomena in cylindrical coordinates, for a thin wide rectangular Newtonian isothermal sheet. We uncover a universal dimensionless relation between sag and time, and a useful dimensionless group that we call sagability. We find that the middle of the sagging sheet, unsuppressed, descends, to leading order, with the cube root of time, and our experiments confirm this. Also, we discover that at a particular time the sag increases rapidly without bound, and we call this phenomenon sag runaway. POLYM. ENG. SCI., 50:2060–2068, 2010. © 2010 Society of Plastics Engineers     

Network numerical simulation of two-dimensional nonlinear micropolar hydrodynamics in a Darcian porous medium

The two-dimensional steady-state boundary layer flow of an incompressible micropolar fluid in a Darcian porous medium is studied theoretically and computationally. The governing parabolic partial differential equations are reduced to dimensionless form by using a set of transformations, under appropriate boundary conditions. A network simulation method (NSM) solution is presented. Translational velocities (U, V) are found to increase with a rise in Darcy number (Da) and to increase and decrease, respectively, with a rise in micropolar parameter (Er), i.e., Eringen number (ratio of micropolar vortex viscosity to Newtonian viscosity). Micro-rotation is increased with increasing Er and Da values. Translational velocity gradient, ∂U/∂Y and micro-rotation gradient, ∂Ω/∂Y both increase with Darcy number; however, they are both found to decrease with increasing micropolar parameter, Er. The present study finds applications in polymer flows in filtration systems, chemical engineering, biorheology of porous tissue and plastic sheet processing.     

The role of process parameters in determining wall thickness distribution in plug‐assisted thermoforming

This article describes the results of a comprehensive investigation to determine the link between process parameters and observed wall thickness output for the plug‐assisted thermoforming process. The overall objective of the work was to systematically investigate the process parameters that may be adjusted during production to control the wall thickness distribution of parts manufactured by plug‐assisted thermoforming. The parameters investigated were the sheet temperature, plug temperature, plug speed, plug displacement, plug shape, and air pressure. As well as quantifying the effects of each parameter on the wall thickness distribution, a further aim of the work was to improve the understanding of the physical mechanisms of deformation of the sheet during the different stages of the process. The process parameters shown to have the greatest effect on experimentally determined wall thickness distribution were the plug displacement, sheet temperature, plug temperature, and plug shape. It is proposed that during the plug‐assisted thermoforming of polystyrene the temperature dependent friction between the plug and sheet surface was the most important factor in determining product wall thickness distribution, whereas heat transfer was shown to play a less important role. POLYM. ENG. SCI., 50:1923–1934, 2010. © 2010 Society of Plastics Engineers     

Composition, Physical Properties and Drying Characteristics of Seed Oil of Momordica charantia Cultivated in Sri Lanka

Karawila (Momordica charantia), also known as bitter gourd, is widely used as a food and a medicine in Asian countries. Representative samples of the seeds of the most abundant cultivar (MC43) in Sri Lanka were collected. The kernel represented 60 ± 4.7% of the seed by dry weight basis. The oil content of the dry kernel was 40.45 ± 3.12%. The seed oil was rich in α-eleosteric acid (50.04 ± 4.80%) and three other geometrical isomers of 9,11,13-octadecatrienoic acid that constituted 6.55%. The acid value, the saponification value and the iodine value were 2.73 ± 0.876, 190.70 ± 1.82 mg/g and 115.96 ± 3.46 cg/g, respectively. The set-to-touch drying time of 3 h observed for the seed oil of MC43 was significantly less than that of linseed oil (13 h). The presence of a high amount of conjugated octadecatrienoic acids, low acid value, high saponification value, moderate iodine value and the low set-to-touch drying time are promising indicators of the potential of karawila seed oil as a good drying oil for the paint and coating industry.     

Novel techniques to investigate the impact of cellulose esters on the rheological properties and appearance in automotive basecoat systems

The viscous response of a coating formulation changes over several orders of magnitude after application onto a substrate. Furthermore, the performance and application attributes for a refinish basecoat formulation are slightly different from those of an OEM basecoat. The absence of a high temperature baking step during the refinish process necessitates that the paint exhibit very good antisag behavior as well as quick ‘dry to touch’ without compromising the flow and leveling properties of the coating as it dries. As most paint formulations are multicomponent in nature, it is always a challenge to precisely measure the impact of specific formulation components on the collective performance attributes of the applied coating. In this work, the focus has been to develop novel techniques that can be used to demonstrate a more quantitative measure of some of the performance attributes that rheological additives like cellulose esters can provide to automotive coatings. A series of pigmented basecoat formulations were prepared which contained cellulose acetate butyrates (CABs) of varying molecular weights at different levels based on the total solids of the coating formulation. The nonvolatile content of the formulations was also varied. The viscoelastic behavior of a typical automotive basecoat formulation during the drying process was then investigated using a novel rheological technique. Complex viscosity data (including storage and loss moduli as well as tan delta) were determined as a function of drying time and then compared to the macroscopic properties typically associated with a coating film as it dries. Thermogravimetric analysis (TGA) was also used to correlate the rheology of the metallic basecoat formulations with drying behavior of the coating. The final appearance of the coating was investigated by several microscopic techniques such as Laser Scanning Confocal Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, and Surface Profilometery, and attempts were made to correlate bulk measurements like ‘flop index’ with the microstructure of the coating. Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006.     

Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed

A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of poly-hydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate (N_c) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 °C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying.     

Restoration of drying cylinders on spinning machines by gas detonation spraying

The possibility of using gas detonation spraying of PR-NKh17SR4 self-fluxing alloy (79% Ni, 15% Cr, 3% Si, 2.4% B) for restoring the drying cylinders on spinning machines was demonstrated. The results of performance tests of drying cylinders with the applied coating at Kamenskvolokno Co. are reported. Translated from Khimicheskie Volokna, No. 6, pp. 43–45, November–December, 2008.     

The effects of radiation cross‐linking and process parameters on the behavior of polyamide 12 in vacuum thermoforming

Compared to amorphous thermoplastics, semi‐crystalline thermoplastics usually have a smaller processing range for thermoforming, due to their narrow temperature window for the transition from viscoelastic to viscous material behavior. On the other hand, semi‐crystalline thermoplastics offer superior properties for applications like ductility or chemical resistance. Within this article, modification of semi‐crystalline polyamide 12 by radiation cross‐linking with respect to its suitability for vacuum thermoforming as well as the effects of processing parameters and sheet thickness on the resulting strain distributions in thermoformed parts are shown. Experimental thermoforming processing studies in combination with digital image correlation measurements, thermo‐mechanical and elongational rheometry were performed to characterize the behavior of cross‐linked semi‐crystalline thermoplastics in the vacuum thermoforming process. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers     

Use of chemical fibres in drying screens for the paper industry

The basic types of chemical fibres for the development and production of drying screens for paper machines were selected and their properties were determined. Polyester fibres are the most appropriate type for fabrication of drying screens due to their elevated mechanical properties and thermohydrolytic stability. Optimum designs for drying screens based on polyester monofilaments and complex fibres with a polymer coating with defined permeability were developed. To decrease the markability of paper web, it was suggested that the outer supporting surface of the screen be coated with soft fibres or a fibrous layer of Nitron fibres. Translated from Khimicheskie Volokna, No. 2, pp. 43–47, March–April, 1998.     

Experimental study on tensioned‐web slot coating

Tensioned‐web slot coating (TWSC) technology was developed in the early 1980s and was considered an efficient method for thin liquid film coating. An experimental study was carried out to investigate the effects of several key parameters on the minimum wet thickness of TWSC. The experiment was performed on a pilot coater with dilute Newtonian poly(vinyl) alcohol (PVA) solutions as test fluids coating on polyethylene‐terephthalate (PET) substrates. It was found that the minimum wet thickness for TWSC was between 0.5 and 1.5 μm, which is much lower than that obtained using conventional slot die coating. The minimum wet thickness was proportional to the tension number T_N, which is defined as the ratio of fluid viscous force over web tension. Furthermore, on the basis of the experimental data, two additional dimensionless groups: dimensionless pressure P_N and dimensionless surface tension L_N were found to be relevant to the performance of TWSC. A universal correlation involving these dimensionless groups as well as the die geometry was used to predict the minimum wet thickness of TWSC. POLYM. ENG. SCI., 47:841–851, 2007. © 2007 Society of Plastics Engineers     

The influence of extrusion parameters on the mechanical properties of polypropylene sheet

Extruded thermoplastic sheet is widely used in the production of thin-gauge tubs and containers for the food and beverage industry using the thermoforming process. The production of high quality thermoformed parts is critically dependent on the standard of extruded sheet feedstock used. The extrusion process itself imparts a thermal history to the sheet, and this in turn partly dictates its subsequent thermoformability. This paper assesses the influence of various extrusion parameters on the mechanical and morphological properties of polypropylene sheet, with a view to defining the optimum extrusion conditions for polypropylene. The extrusion parameters under consideration are chill-roll temperature, line speed, sheet thickness and melt temperature.     

Drying and collapse of hollow latex

Hollow latex particles are used as white pigments for paints and paper coatings. In the coating dispersion, each hollow particle is filled with water. As the coating dries, water vacates the latex, leaving an air-filled void sized to scatter light (~0.5 μm) within each particle. Examinations of dried coatings reveal that hollow particles can collapse, decreasing their light scattering efficiency. Cryogenic scanning electron microscopy (cryoSEM) was used to characterize the microstructure of coatings containing hollow latex during drying. Images suggest latex voids empty after air invades into the coating interstitial space and collapse occurs late in the drying process. The effects of temperature (10–60°C), humidity (20–80%), and binder concentration (0–30 wt%) on particle collapse were also studied through SEM of dried coating surfaces. High drying temperature, high humidity, and low binder concentrations promoted collapse. For hollow latex particles with porous shell walls, temperature and humidity had little effect, whereas binder increased collapse. From these results, a theoretical model is proposed. During drying, diffusion of water from the particle creates a vacuum inside the latex. The vacuum is either relieved by nucleation of a gas bubble from the dissolved air in the water-filled particle or it causes the particle to collapse by buckling.     

A Generalized Dimensionless Model for Deep Bed Drying of Paddy

A generalized dimensionless model of paddy drying was developed from a validated partial differential equation (PDE) drying model using the dimensional analysis of Buckingham theorem. This generalized dimensionless model considered all drying parameters in an equation to predict the grain moisture content during the drying process. Statistical parameters, namely, coefficient of determination (R ²), chi-square (χ²), mean relative deviation (MRD), and root mean square error (RMSE), were used as criteria to compare the dimensionless model with a validated PDE model. Based on these calculated parameters, it was concluded that the generalized dimensionless model fitted reasonably well with data from the PDE model and good agreement was found between the generalized dimensionless model and experimental drying data.     

Impact of polymer processing on sorption of benzaldehyde vapor in amorphous and semicrystalline polypropylene

In this study, organic vapor sorption properties of different polypropylene (PP) forms (resin, sheet, thermoformed sheet, and atactic PP) at 25°C were investigated. The sorption of benzaldehyde at five vapor activities was studied by using a continuous gravimetric method. The sorption of benzaldehyde vapor in PP exhibited different behaviors among resin, sheet, and polyhedron thermoformed sheet. The experimental results demonstrated that the extrusion and thermoforming processes used for forming these parts had a great impact on the sorption behavior because of the changes in surface and bulk morphologies, especially at high vapor activities. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Flexible blade coating

Flexible blades are often employed to spread liquid coating thinly onto a flat smooth substrate. In this study, we derive a fifth-order nonlinear ordinary differential equation for the thickness of a Newtonian coating and for the corresponding blade deflection. After solving this equation numerically, a graph is produced to help engineers predict the coating thickness. We find that blade deflection and coating thickness are governed by the blade angle and by a new dimensionless group called the blade flexibility. For values of blade flexibility less than one, the coating thickness matches that of a rigid blade. The results of this analysis provide an engineer with the ability to design a flexible blade configuration to deliver the desired coating thickness onto a substrate.