Viscous State Research Articles

Modeling of the plywood making process glued with thermoplastic polymer

A promising variety of the wood composite materials is plywood glued with thermoplastic polymers. However, making such a wood composite requires adjustment of the pressing modes, because there is a slightly difference between the hot method of the veneer packages pressing glued with thermoplastic films and the method of the veneer packages pressing glued with thermosetting adhesives. Among the main processes that occur in the gluing plywood process are heat and mass transfer. The availability of the thermoplastic film between the veneer sheets is going to change the thermal conductivity of the package, and therefore will affect the pressing duration. If suppose, that the package has a plate appearance, which is located between heated to the certain temperature pressed plates, then the known Fourier-Kirchhoff thermal differential equation can be applied. The basis for the model development of the warming package veneer process glued with thermoplastic film are laid methods of nonlinear programming, uniform search and conjugate gradient. On the offered mathematical model basis, the temperature field change in the package thickness during the plywood gluing with plastic film is calculated, the calculation of the warming duration indicators of the package veneer is done and this indicator dependence on the thermoplastic film flow rate and the pressing temperature is established. The duration of the warming package veneer glued with thermoplastic film depends on the temperature at which the thermoplastic polymer will change from a highly elastic to the viscous state. The transition of the thermoplastic film LDPE to the viscous state begins at 125оС and lasts up to 240 оС. With increasing of the press temperature plates from 140 to 180оС the duration of warming up the middle of the package to 125оС decreases by 89% for all investigated polymer costs is established. Changing the polymer content in the package does not significantly affect the duration of its warming. With increasing of the thermoplastic film flow rate from 130 g/m2 to 190 g/m2, the duration of warming up the middle of the veneer package to 125оС increases slightly from 3,8 to 4,2% depending on the pressing temperature. Suggested mathematical model allows to set the material parameters and plywood pressing mode influence on the duration of the warming package veneer while plywood making glued with the thermoplastic film LDPE.

Temperature Effects on the Dielectric Properties and Breakdown Performance of h-BN/Epoxy Composites.

Epoxy–boron nitride composites are promising insulating materials, and it is highly important to understand their insulating performances at different temperatures with different nano-doping amounts. In this study, we investigated the effects of different mass fractions of epoxy–micron hexagonal boron nitride composites on their thermal conductivity, as well as the effects of temperature and mass fraction on their insulating performances. The results demonstrated that the thermal conductivity of epoxy–micron hexagonal boron nitride composites was superior to that of neat epoxy. The thermal conductivity of epoxy–micron hexagonal boron nitride composites increased with the mass fraction of hexagonal boron nitride, and their dielectric constant and dielectric loss increased with temperature. The dielectric constant of epoxy–micron hexagonal boron nitride composites decreased as the mass fraction of hexagonal boron nitride increased, while their dielectric losses decreased and then increased as the mass fraction of hexagonal boron nitride increased. Due to internal heat accumulation, the alternating current breakdown strength of epoxy–micron hexagonal boron nitride composites increased and then decreased as the mass fraction of hexagonal boron nitride increased. Additionally, as the temperature increased, the composites transitioned from the glassy state to the rubbery or viscous state, and the breakdown strength significantly degraded.

Rheological, Thermal, Superficial, and Morphological Properties of Thermoplastic Achira Starch Modified with Lactic Acid and Oleic Acid.

The modification of achira starch a thermoplastic biopolymer is shown. Glycerol and sorbitol, common plasticizers, were used in the molten state with organic acids such as oleic acid and lactic acid obtaining thermodynamically more stable products. The proportion of starch:plasticizer was 70:30, and the acid agent was added in portions from 3%, 6%, and 9% by weight. These mixtures were obtained in a torque rheometer for 10 min at 130 °C. The lactic acid managed to efficiently promote the gelatinization process by increasing the available polar sites towards the surface of the material; as a result, there were lower values in the contact angle, these results were corroborated with the analysis performed by differential scanning calorimetry and X-ray diffraction. The results derived from oscillatory rheological analysis had a viscous behavior in the thermoplastic starch samples and with the presence of acids; this behavior favors the transitions from viscous to elastic. The mixture of sorbitol or glycerol with lactic acid promoted lower values of the loss module, the storage module, and the complex viscosity, which means lower residual energy in the transition of the viscous state to the elastic state; this allows the compounds to be scaled to conventional polymer transformation processes.

Study the safeguarding performance of shear thickening gel by the mechanoluminescence method

This work reports the potential of a full-field, simple-setup and user-interactive method to study the safeguarding property of shear thickening gel (STG) by converting the invisible force/energy information to visible mechanoluminescence. Both the instantaneous intensity and force signals under impact prove the phase change of STG from viscous liquid state to rubbery state. The illumination images, presenting the energy maps, directly visualize the remarkably expanded impact area and phase change induced energy absorption, promoting the understanding of safeguarding mechanisms. The shear thickening property and viscoelastic deformation of STG significantly expand the impact area by almost 5 times, spreading and dissipating force/energy significantly. Moreover, the phase change accompanied by cracks is observed clearly, which is demonstrated to be beneficial to the energy absorption and contributes to the further enlargement of the impact area.

Neural network-based model of photoresist reflow

When polymers are heated above their glass transition temperature, they enter a viscous rubbery state that allows the polymer to be reshaped in a process called “reflow.” The final shape depends on the material, substrate, the initial dimensions of the structure, the reflow temperature, and time and is mostly governed by energy minimization. Most empirical models so far have used linear regression to predict scalar parameters like the thickness of the reflowed structure but do not account for intermediate shapes. In this work, the authors measure the profiles of photoresist patterns subjected to various reflow conditions, complementing results in the literature. Using shallow neural networks, they develop models to predict the type of shape produced after reflow and its full cross-sectional height profile. These models can serve as an aid for polymer engineering and fabrication and also demonstrate the usefulness of a neural network-based approach to physical optimization problems without analytical solutions.

Clever use of PCL as kinetic controller in PBT/PCL blends

This work investigated the melting and nonisothermal melt crystallization of poly (butylene terephthalate) (PBT) and poly (ε-caprolactone) (PCL) blends with PCL content ranging from 3 to 10% of the weight. Thermal stability and morphology analyses were performed by thermogravimetry (TG) and scanning electron microscopy (SEM), whereas the nonisothermal crystallization of PBT/PCL blends carried out by differential scanning calorimetry (DSC), and the kinetic parameters were determined using Pseudo-Avrami, Ozawa and Mo models. PCL improved thermal stability by decreasing the degradation rate of PBT. From SEM images the brittle fracture of PBT was modified through PCL addition with absorption energy mechanisms developed in PBT/PCL blends. PCL delayed the melt crystallization of PBT, most due to its fluid viscous state during PBT crystallization. Ozawa model failed to provide an effective description of the blends, while Pseudo-Avrami and Mo models successfully described the nonisothermal melt crystallization, and linearity deviations were observed only at the beginning and at the end of crystallization, probably due to nucleation and spherulite impingement during secondary crystallization.

Centre modes in pipe flow

Abstract In two previous papers, Ozcakir, Tanveer, Hall, & Overman (2016, Travelling waves in pipe flow, J. Fluid Mech., 791, 284–328) and Ozcakir, Hall & Tanveer (2019, Nonlinear exact coherent structures in pipe flow and their instabilities, J. Fluid Mech., 868, 341–368) investigated numerically and asymptotically high Reynolds number exact coherent structures in pipe flow. It was found that, in addition to the structures described by the vortex–wave interaction theory by Hall & Smith (1991, On strongly nonlinear vortex/wave interactions in boundary layer transition. J. Fluid Mech., 227, 641–666), there exists vortical structures localized near the centre of the pipe with a core of size $O(Re^{-1/4})$ convected downstream at a speed that deviates from the pipe centreline speed by $O(Re^{-1/2})$, where $Re$ is the Reynolds number. In the finite Reynolds number calculations by Ozcakir, Tanveer, Hall & Overman (2016, Travelling waves in pipe flow, J. Fluid Mech., 791, 284–328), asymptotic state was referred to as a nonlinear viscous core state (NVC). However the reduced asymptotic equations were not solved and only limited confirmation of the theory was found numerically. Here, in order to conclusively confirm the existence of the NVC state we first describe direct numerical calculations on the asymptotically reduced $Re>>1$ equations for such state states. The results are then compared in detail to the finite $Re$ calculations up-to $Re=10^6$; the latter regime is at much higher values of the Reynolds number than those reported in Ozcakir, Tanveer, Hall & Overman (2016, Travelling waves in pipe flow, J. Fluid Mech., 791, 284–328). The results are found to be in excellent agreement with the finite $Re$ calculations in a region between $Re=10^5$ and $10^6$, thereby confirming that the structure observed by Ozcakir, Tanveer, Hall & Overman (2016, Travelling waves in pipe flow, J. Fluid Mech., 791, 284–328) is indeed a finite Reynolds number realization of an asymptotic NVC state.

Hydrodynamics of the bladderwort feeding strike.

The aquatic bladderwort Utricularia gibba captures zooplankton in mechanically triggered underwater traps. With characteristic dimensions <1 mm, the trapping structures are among the smallest known that work by suction-a mechanism that would not be effective in the creeping-flow regime. To understand the adaptations that make suction feeding possible on this small scale, we have measured internal flow speeds during artificially triggered feeding strikes in the absence of prey. These data are compared with complementary analytical models of the suction event: an inviscid model of the jet development in time and a steady-state model incorporating friction. The initial dynamics are well described by a time-dependent Bernoulli equation in which the action of the trap door is represented by a step increase in driving pressure. According to this model, the observed maximum flow speed (5.2 m/s) depends only on the pressure difference, whereas the initial acceleration (3 × 104 m/s2 ) is determined by pressure difference and channel length. Because the terminal speed is achieved quickly (~0.2 ms) and the channel is short, the remainder of the suction event (~2.0 ms) is effectively an undeveloped viscous steady state. The steady-state model predicts that only 17% of power is lost to friction. The energy efficiency and steady-state fluid speed decrease rapidly with decreasing channel diameter, setting a lower limit on practical bladderwort size.

A Self-Healing and Shape Memory Polymer that Functions at Body Temperature.

Dual-functional polymeric system combining shape memory with self-healing properties has attracted increasingly interests of researchers, as both of these properties are intelligent and promising characteristics. Moreover, shape memory polymer that functions at human body temperature (37 °C) are desirable because of their potential applications in biomedical field. Herein, we designed a polymer network with a permanent covalent crosslinking and abundant weak hydrogen bonds. The former introduces elasticity responsible and maintain the permanent shape, and the latter contributes to the temporary shape via network rearrangement. The obtained PDMS-COO-E polymer films exhibit excellent mechanical properties and the capability to efficiently self-heal for 6 h at room temperature. Furthermore, the samples turn from a viscous state into an elastic state at 37 °C. Therefore, this polymer has shape memory effects triggered by body temperature. This unique material will have a wide range of applications in many fields, containing wearable electronics, biomedical devices, and 4D printing.

Viscous dissipation in two-dimensional compression of turbulence

Nonradial hydrodynamic flow can be generated or amplified during plasma compression by various mechanisms, including the compression itself. In certain circumstances, the plasma may reach a viscous state; for example, in compression experiments seeking fusion, the fuel plasma may reach a viscous state late in the compression due in part to the rising fuel temperature. Here, we consider viscous dissipation of nonradial flow in the case of initially isotropic, three-dimensional (3D), turbulent flow fields compressed at constant velocity in two dimensions. Prior work in the case of 3D compressions has shown the possibility of effective viscous dissipation of nonradial flow under compression. We show that, theoretically, complete viscous dissipation of the nonradial flow should still occur in the 2D case when the plasma heating is adiabatic and the viscosity has the (strong) Braginskii temperature dependence (μ∼T5/2). However, in the general case, the amount of compression required is very large even for modest initial Reynolds numbers, with the compression reaching an intermediate state dominated by variations only in the noncompressed direction. We show that both the nonlinearity and boundary conditions can play important roles in setting the characteristics and ease of the viscous dissipation.

A Novel Route to Fabricate High-Performance 3D Printed Continuous Fiber-Reinforced Thermosetting Polymer Composites.

Recently, 3D printing of fiber-reinforced composites has gained significant research attention. However, commercial utilization is limited by the low fiber content and poor fiber–resin interface. Herein, a novel 3D printing process to fabricate continuous fiber-reinforced thermosetting polymer composites (CFRTPCs) is proposed. In brief, the proposed process is based on the viscosity–temperature characteristics of the thermosetting epoxy resin (E-20). First, the desired 3D printing filament was prepared by impregnating a 3K carbon fiber with a thermosetting matrix at 130 °C. The adhesion and support required during printing were then provided by melting the resin into a viscous state in the heating head and rapidly cooling after pulling out from the printing nozzle. Finally, a powder compression post-curing method was used to accomplish the cross-linking reaction and shape preservation. Furthermore, the 3D-printed CFRTPCs exhibited a tensile strength and tensile modulus of 1476.11 MPa and 100.28 GPa, respectively, a flexural strength and flexural modulus of 858.05 MPa and 71.95 GPa, respectively, and an interlaminar shear strength of 48.75 MPa. Owing to its high performance and low concentration of defects, the proposed printing technique shows promise in further utilization and industrialization of 3D printing for different applications.

Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest.

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas-particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

A novel composite silicone foam with enhanced safeguarding performance and self-healing property

A novel hybrid silicone foam was successfully prepared by mixing shear stiffening gel (STG) with polymethylvinylsiloxane (MVQ) using expandable microsphere as a blowing agent. Storage modulus of STG/MVQ composite foam increased from 0.045 MPa to 0.38 MPa, when the shear frequency shifted from 0.1 Hz to 100 Hz, presenting a remarkable shear stiffening characteristic. The tensile result demonstrated that the elongation at break of the hybrid foam was as high as 814%. Besides, the hybrid foam gained excellent self-healing performance due to the cold-flow feature and viscous state of STG. More importantly, the drop hammer test indicated the safeguarding performance enhanced as the STG contents increased, with the residual peak force decreasing from 36.72 KN to 31.50 KN. In addition, a possible mechanism was proposed that it was the dynamic “BO bonds” that played a significant role in shear stiffening.

Impact of gas composition on thermal conductivity of glass foams prepared via high-pressure sintering

Abstract Glass foam is a cellular material with excellent thermal insulating ability. The thermal conductivity (λ) of cellular materials is complex to understand because it is determined by the composition of the solid and gas phases, pore size, solid microstructure (amorphous or crystalline, struts, and walls), and radiation. In this paper, we focus on the influence of gas composition on λ of glass foams. Glass foams were prepared by a physical foaming approach, where powder pellets of cathode ray tube (CRT) panel glass were sintered at 640 °C in Ar or N2 atmosphere at elevated pressure (5–25 MPa), and then cooled down to room temperature. When heating the sintered samples to the viscous state (approx. 108–106 Pa s corresponding to 650–740 °C), the high internal gas pressure in the closed pores expands the glass melt. We heat-treated each sintered sample multiple times in air at atmospheric pressure to gradually decrease the density, and thereby to obtain multiple λ values of each sample at different densities, while the chemistry of the sample remained almost identical. Gas chromatography revealed binary Ar-CO2 and N2-CO2 gas compositions in the Ar- and N2-sintered samples, respectively. The thermal conductivities of these gas mixtures (λgas,mix) were calculated to be 16.4 and 23.1 mW m−1 K−1, presenting 22 and 31% of the effective λ for the Ar- and N2-sintered samples, respectively. The lower λgas,mix resulted in a lower λ of the Ar-sintered samples compared to the N2-sintered ones at low density (

The Effect of the Thermosensitive Biodegradable PLGA⁻PEG⁻PLGA Copolymer on the Rheological, Structural and Mechanical Properties of Thixotropic Self-Hardening Tricalcium Phosphate Cement.

The current limitations of calcium phosphate cements (CPCs) used in the field of bone regeneration consist of their brittleness, low injectability, disintegration in body fluids and low biodegradability. Moreover, no method is currently available to measure the setting time of CPCs in correlation with the evolution of the setting reaction. The study proposes that it is possible to improve and tune the properties of CPCs via the addition of a thermosensitive, biodegradable, thixotropic copolymer based on poly(lactic acid), poly(glycolic acid) and poly(ethylene glycol) (PLGA–PEG–PLGA) which undergoes gelation under physiological conditions. The setting times of alpha-tricalcium phosphate (α-TCP) mixed with aqueous solutions of PLGA–PEG–PLGA determined by means of time-sweep curves revealed a lag phase during the dissolution of the α-TCP particles. The magnitude of the storage modulus at lag phase depends on the liquid to powder ratio, the copolymer concentration and temperature. A sharp increase in the storage modulus was observed at the time of the precipitation of calcium deficient hydroxyapatite (CDHA) crystals, representing the loss of paste workability. The PLGA–PEG–PLGA copolymer demonstrates the desired pseudoplastic rheological behaviour with a small decrease in shear stress and the rapid recovery of the viscous state once the shear is removed, thus preventing CPC phase separation and providing good cohesion. Preliminary cytocompatibility tests performed on human mesenchymal stem cells proved the suitability of the novel copolymer/α-TCP for the purposes of mini-invasive surgery.

Time evolution of steep diffusion fronts in highly viscous aerosol particles measured with Mie resonance spectroscopy.

Field and laboratory measurements indicate that atmospheric organic aerosol particles can be present in a highly viscous state. In contrast to liquid state particles, the gas phase equilibration to ambient relative humidity (RH) can be kinetically limited and governed by condensed phase diffusion. In water diffusion experiments on highly viscous single aerosol particles levitated in an electrodynamic balance, we observed a characteristic shift behavior of the Mie scattering resonances indicative of the changing radial structure of the particle, thus providing an experimental method to track the diffusion process inside the particle. Due to the plasticizing effect of water, theory predicts extremely steep, front-like water concentration gradients inside highly viscous particles exposed to a rapid increase in RH. The resulting quasi step-like concentration profile motivates the use of a simple core-shell model describing the morphology of the non-equilibrium particle during humidification. The particle growth and reduction of the shell refractive index can be observed experimentally as redshift and blueshift behavior of the Mie resonances, respectively. We can deduce the particle radius as well as a core-shell radius ratio from the measured shift pattern and Mie scattering calculations. Using both the growth information obtained from the Mie resonance redshift and thermodynamic equilibrium data, we can infer a comprehensive picture of the time evolution of the diffusion fronts in the framework of our core-shell model. The observed shift behavior of the Mie resonances provides direct evidence of very steep diffusion fronts caused by the plasticizing effect of water and a method to validate previous diffusivity measurements.

Prediction of core deflection in wax injection for investment casting by using SVM and BPNN

Core shift is a common basic problem during the manufacture of those thin-walled hollow casting parts in investment casting. The core deflection during wax injection is the main reason for exceeding tolerance in final cast. The wax melt may advance asymmetrically along both sides of the core in wax injection process for a thin-walled hollow complex part, which causes the core to shift, resulting in non-uniform wall thickness of the wax pattern. The simulation results and experimental measurements of core deflection on the wax pattern for a hollow pump body were studied. The SVR (support vector regression) has been used to predict the core deflection for the wax pattern. Packing pressure, injection temperature and injection velocity were chosen as main process parameters. The core deflection of the wax pattern is most influenced by the melt temperature because of larger stress distribution acting on the cores in the viscous state at low injection temperature. SVR and BPNN (back-propagation neural network) were used to establish the prediction model; SVR has a better predictive ability compare with BPNN.

Cure kinetics and viscosity modeling for the optimization of cure cycles in a vacuum-bag-only prepreg process

This study focuses on the development of a process-based simulation model coupled with differential scanning calorimetry (DSC) and dynamic mechanical analyzer (DMA) experiments. The cure kinetics and rheology of an epoxy-amine resin were characterized in order to predict the degree of cure and viscosity behavior in an out-of-autoclave (OOA) process condition. Both phenomenological reactions and chemo-rheological models were applied to effectively predict the degree of cure and resin viscosity. Using these results, it is possible to predict the minimum viscosity, gelation, and vitrification, which are the main process factors of the multi-stepped cure cycles, and reduce the number of process trials. It was found that there was a good correlation between the experimental results and model predictions under isothermal and non-isothermal temperature profiles. Furthermore, this study demonstrates that there is an additional scope for optimization in the conventional cure cycles recommended by prepreg manufacturers, especially when a low viscous state is required. The optimized cure cycle led to a substantially high fiber fraction (58.96 vol%) and a low void content (0.15 vol%), as compared to the conventional cure cycles.

High temperature polymerization monitoring of an epoxy resin using ultrasound

In this study, the real time ultrasonic monitoring is investigated to quantify changes in physical and mechanical properties during the manufacture of composite structures. In this context, an experimental transmission was developed with the aim of characterizing a high temperature polymerization reaction and post-curing properties using an ultrasonic method. First, the monitoring of ultrasonic parameters of a thermosetting resin is carried out in a device reproducing the experimental conditions for manufacturing a composite material with a process known as RTM, that is to say an isothermal polymerization at T = 160°C. During this curing, the resin is changing from its initial viscous liquid state to its final viscous solid state. Between those states, a glassy transition stage is observed, during which the physical properties are strongly changing, i.e. an increase of the ultrasonic velocity up to its steady value and a transient increase of the ultrasonic attenuation. Second, the ultrasonic inspection of the thermosetting resin is performed during a heating and cooling process to study the temperature sensitivity after curing. This type of characterization leads to identifying the ultrasonic properties dependence before, during and after the glassy transition temperature Tg. Eventually, this study is composed of two complementary parts: the first is useful for the curing optimization, while the second one is fruitful for the post-processing characterization in a temperature range including the glassy transition temperature Tg.

A particle-size regulated approach to producing high strength gasification-coke by blending a larger proportion of long flame coal

To produce a qualified gasification-coke by blending a larger proportion of low-rank coal, the effects of blending ratio, particle size distribution, and maceral components on the coke strength were studied, and a new coking mechanism was put forward in terms of the physical texture. The results show that the influence of coal particle size varies with the different cohesiveness of coals. Fat coal (FC) with a smaller size gets a better performance on the coke strength, while a larger size is needed for gas coal (GC), and the optimum size of long flame coal (Shenfu coal, SFC) varied with its blending ratio. SFC and GC should be crushed coarsely (2–3 mm surpasses 40%) to act as the framework with the less external surface in the viscous embedded state. FC should be crushed finely (<0.2 mm) to form flowing metaplast that being filled uniformly in the gap between these coarse particles, thus reinforcing the embedded state and leading to a high coke strength with a less blend of FC (25 wt%). Moreover, liptinite enriched in the coarse particles of GC and SFC improves the cohesiveness of blended coal moderately. A qualified gasification-coke (M25 > 90%, CRI > 50%, CSR > 35%) can be obtained by blending >40 wt% SFC with this new coking mechanism.