Shrinkage Kinetics Research Articles

Thermogelable PNIPAM microgel dispersion as 3D cell scaffold: effect of syneresis

Using poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (P(NIPAM-HEMA)) microgel as an example, we proposed that thermogelable microgel dispersions can be used as a new type of injectable cell scaffolds. However, these in situ formed hydrogels shrink with time, which is undesirable for their use as cell scaffolds. In this work, the syneresis of hydrogels from poly(N-isopropylacrylamide) (PNIPAM) microgel dispersions with various acrylic acid (AA) contents were measured. These concentrated microgel dispersions can all thermally gelate at physiological pH and ionic strength. With increasing AA content in the microgels, the gelation temperature of the dispersion increases, but the degree of syneresis of the resulting hydrogels decreases. The kinetics of shrinkage can be well described by the Tanaka-Fillmore equation, indicating that the deswelling of the hydrogels is governed by the cooperative diffusion of the gel network. These hydrogels present an interconnected porous structure with pore size decreases with an increasing degree of syneresis. HepG2 cells were seeded in these hydrogels. While the cells proliferate and form spheroid-like aggregates in hydrogels with a low degree of syneresis, their growth is inhibited in the one with a high degree of syneresis. These results indicate that the syneresis of the hydrogel has an adverse effect on cell culturing. This effect can be alleviated by adjusting AA content in the microgels. However, cells do not grow either if AA content is too high which results in a lack of cell-substrate interaction.

Polymerization shrinkage kinetics of silorane-based composites

Dental composites have improved significantly in physical properties over the past few decades. However, polymerization shrinkage and stress is still the major drawback of composites, limiting its use to selected cases. Much effort has been made to make low shrinking composites to overcome this issue and silorane-based composites have recently been introduced into the market. The aim of this study was to measure the volumetric polymerization shrinkage kinetics of a silorane-based composite and compare it with conventional methacrylate-based composites in order to evaluate its effectiveness in reducing polymerization shrinkage. Five commercial methacrylate-based (Beautifil, Z100, Z250, Z350 and Gradia X) and a silorane-based (P90) composites were investigated. The volumetric change of the composites during light polymerization was detected continuously as buoyancy change in distilled water by means of Archemedes' principle, using a newly made volume shrinkage measurement instrument. The null hypothesis was that there were no differences in polymerization shrinkage, peak polymerization shrinkage rate and peak shrinkage time between the silorane-based composite and methacrylate-based composites. The results were as follows: 1. The shrinkage of silorane-based (P90) composites was the lowest (1.48%), and that of Beautifil composite was the highest (2.80%). There were also significant differences between brands among the methacrylate-based composites. 2. Peak polymerization shrinkage rate was the lowest in P90 (0.13%/s) and the highest in Z100 (0.34%/s). 3. The time to reach peak shrinkage rate of the silorane-based composite (P90) was longer (6.7 s) than those of the methacrylate-based composites (2.4-3.1 s). 4. Peak shrinkage rate showed a strong positive correlation with the product of polymerization shrinkage and the inverse of peak shrinkage time (R = 0.95).

A new method to measure the linear polymerization shrinkage of composites using a particle tracking method with computer vision

Since the introduction of restorative dental composites, their physical properties have been significantly improved. However, polymerization shrinkage is still a major drawback. Many efforts have been made to develop a low shrinking composite, and silorane-based composites have recently been introduced into the market. In addition, many different methods have been developed to measure the polymerization shrinkage. In this study, we developed a new method to measure the linear polymerization shrinkage of composites without direct contact to a specimen using a particle tracking method with computer vision. The shrinkage kinetics of a commercial silorane-based composite (P90) and two conventional methacrylate-based composites (Z250 and Z350) were investigated and compared. The results were as follows: 1. The linear shrinkage of composites was 0.33-1.41%. Shrinkage was lowest for the silorane-based (P90) composite, and highest for the flowable Z350 composite. 2. The new instrument was able to measure the true linear shrinkage of composites in real time without sensitivity to the specimen preparation and geometry. [J Kor Acad Cons Dent 35(3):180-187, 2010]

Polymerization shrinkage kinetics of dimethacrylate resin-cements

Objectives To determine polymerization shrinkage-strain ( S Y) and shrinkage-stress ( S Z) of six resin-cements and to compare their performance with the aid of degree of conversion (DC) data. Methods Variolink 2 (VL2), Multilink Automix (MA), Multilink Sprint (MS, all Ivoclar-Vivadent), Nexus 2 (NX2), Maxcem (MX, both Kerr) and RelyX Unicem (RX, 3M-Espe) were investigated. MS, MX and RX were self-adhesive; others require a bonding-agent. All measurements were conducted at 23 °C for 60 min ( n = 5), except 80 min for RX, with materials self-cured only (sc) and dual-cured (dc); NX2 and VL2 were additionally light-cured only (lc). S Y was measured by the bonded-disk method [Watts DC, Cash AJ. Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 1991;7(4):281–7; Watts DC, Marouf AS. Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials. Dent Mater 2000;16(6):447–51]; S Z by the Bioman instrument [Watts DC, Satterthwaite JD. Axial shrinkage-stress depends upon both C-factor and composite mass. Dent Mater 2008;24(1):1–8 [Epub October 24, 2007]; Watts DC, Marouf AS, Al-Hindi AM. Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development. Dent Mater 2003;19(1):1–11]. Light-cure was achieved by QTH at 500 mW/cm 2. The respective DCs were measured under the same conditions by FTIR–ATR spectroscopy. Data were analyzed by One-Way ANOVA plus Bonferroni test, and by t-test, at p < 0.05. Results DC by self-curing was less than the DC by dual-curing, for all cements. Shrinkage-strain ranged from 1.77 to 5.29% and shrinkage-stress from 3.36 to 10.37 MPa. NX2 and VL2 were not significantly different, when light-cured only. Except for RX, sc and dc shrinkage-strain varied maximally by 0.4%. MX showed the highest S Y, RX the lowest. When sc, RX initially expanded by <0.5% ( t ∼ 5 min). For most materials, S Y correlated with their filler loading. The highest stress with sc was exerted by MX, and when dc by MS, which was not statistically different from MX. Significance Shrinkage data of resin-cements are of intrinsic clinical importance. Self-cure, despite a lower DC, did not necessarily result in a lower S Y compared to dual-cure. S Y-rate and S Z development depend upon cure mode and S Y upon filler fraction.

Shrinkage Kinetics during Convective Drying of Selected Berries

Drying and shrinkage kinetics of gooseberry, black and red currants, cherry, and blackberry were studied using a fully automated experimental apparatus, designed for continuous monitoring of the material moisture content, its temperature, and surface area. Experimental drying curves were approximated with a modified exponential model. The volume and surface area of berries were determined from the pictures taken during drying and transformed into digital images. The procedure for calculation of the object-related pixels in a two-dimensional plane to the temporal three-dimensional volume and surface area of the berries was developed. The modified exponential model was proposed to correlate the volumetric and surface area shrinkage with moisture content. Volumetric shrinkage at the end of drying was maximal (0.3) for black currant and minimal (0.45) for gooseberry.

Forced-Air Drying of Ginseng Root: Pilot-Scale Control System for Three-Stage Process

Pilot-scale experiments were conducted to evaluate a three-stage temperature control system (38–50–38°C) for ginseng root drying. Feedback for determining temperature change points was provided by measuring root shrinkage in digital images of the root bed. Shrinkage kinetics observed during the first stage were used to predict the drying rate for the third stage and an exponential model was used to predict the time required for the roots to reach an average moisture content of 0.11 g/g (db). Based on experimental tests, the root mean square error for the final moisture was 0.025 g/g (db).

Investigation of sintering mechanisms of alumina using kinetic field and master sintering diagrams

Strain rates during sintering of alumina at different heating rates between 0.2 and 20 K/min have been measured non-contact using a thermooptical measuring technique. The shrinkage data were represented in a kinetic field diagram. It was shown that coarsening effects can be indirectly derived from the position and slope of iso-strain lines identified in the kinetic field. In addition temperature gradients which frequently lead to an erroneous interpretation of shrinkage kinetics were identified. Uni-axial loads up to 6 MPa were applied during sintering. The strain rates were measured parallel and perpendicular to the load. They were processed in separate kinetic field diagrams. It was shown that creep leads to a parallel shift of the iso-strain lines either to lower temperatures, for the strain rates parallel to the load, or to higher temperatures, for the strain rates perpendicular to the load. The individual kinetic field diagrams for a specific load were assembled in 3d, with the load used as z-axis. They formed a master sintering diagram (MSD) where the iso-strain lines were replaced by iso-strain surfaces. The parallel shift of the iso-strain lines with load enabled a simple representation of sintering kinetics for various loads during the entire range of densification, temperatures and heating rates.

Experimental determination and modelling of size variation, heat transfer and quality indexes during mushroom blanching

Process time during mushrooms blanching is determined either by shrinking rate or by polyphenoloxidase (PPO) inactivation, depending on the value of heating bath temperature. Quality losses during blanching can be minimized through an adequate selection of the time–temperature schedule. In this work shrinkage and PPO enzymatic activity of mushroom during blanching were measured. A simple kinetics of mushroom shrinkage was developed and was coupled to a heat transfer model based on the finite element method with irregular domain. The effect of this process on the textural and colour quality was experimentally studied and compared with the cooking value predicted from temperature simulations. Shrinkage was found to be the limiting factor to estimate the processing time for bath temperature higher than 60 °C; while polyphenoloxidase activity reduction was the limiting factor at lower bath temperatures. Quality indexes diminished with thermal treatment. Texture measures were in good agreement with simulated cooking values. This last parameter can be optimized by selecting the most appropriate conditions for the blanching process.

Video-controlled characterization of polymerization shrinkage in light-cured dental composites

This study investigated a procedure to measure the intrinsic polymerization contraction of dental resin-based materials. A CCD-camera interfaced with a computer through video-interface hardware measured the coordinates of the centers of gravity of two dot markers stuck on a resin composite cylinder before and after the irradiation. The distances of the centers of gravity, and hence the linear percentage shrinkage, were calculated. The values obtained were compared with those measured by the deflecting disk technique [D.C. Watts, A.J. Cash, Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development, Dent. Mater. 7 (1991) 281–287.], which assessed polymerization shrinkage through deflection of a glass disk measured by an linear vertical displacement transducer (LVDT). Three resin composites, light-cured for 40 s at 800 mW cm −2, were tested: Solitaire 2 [SOL2], Tetric Ceram [TECE] and Z100 [Z100]. The data were subjected to the Kruskal–Wallis test ( p < 0.05). Whatever the experimental procedure, the three resin composites did not present statistically different polymerization shrinkage, and data obtained with the video-controlled technique were statistically higher than with the deflecting disk method.

Influence of ceramic thickness and curing mode on the polymerization shrinkage kinetics of dual-cured resin cements

Objectives The purpose of this study was to assess how ceramic disc thickness and curing mode (light or chemical) affects the polymerization shrinkage of dual-cured resin cements and to evaluate the effect of the ceramic discs on the curing speed of the cements during light exposure. Methods Six commercial resin cements, RelyX ARC, Bistite II, Duolink, Panavia F, Variolink II and Choice were used. Filler weight contents were determined by the ash method. Four ceramic discs with thicknesses of 0.5, 1, 2 and 4 mm, respectively, were made. The attenuation of light intensity due to the ceramic discs was measured using a radiometer. The polymerization shrinkage kinetics of the resin cements by chemical or light cure through the different ceramic discs was measured using a bonded-disc method. Results There were differences in filler content among brands of resin cement. The polymerization shrinkage without ceramic disc was 2.61–4.59% by chemical cure and 2.93–4.66% by light cure. The polymerization shrinkage of RelyX ARC and Panavia F by chemical cure was statistically lower than by light cure ( p < 0.05). Polymerization shrinkage and filler weight were inversely related ( R = −0.965). Both the transmitted light intensity and polymerization shrinkage decreased with increasing thickness of ceramic discs ( p < 0.05). The time to reach the maximum shrinkage rate of the resin cements increased with increasing ceramic thickness. The cure speed by light cure was 15–322 times faster than by chemical cure. Significance The polymerization shrinkage kinetics of dual-cured resin cements significantly differed between brands under various curing conditions. Clinicians should be aware of the setting characteristics of the cements, so they can choose the optimal materials for different clinical situations.

Shrinkage kinetics and thermal behaviour of injection moulded polymers

We measure the kinetics of shrinkage at the time of the cooling of injected polymers during a cycle of moulding. Dynamic measurement of the shrinkage of the castings after ejection is taken by the method of photomechanical in a thermally controlled environment representative of a workshop production one. The technique of photomechanical used allows measurement without contact of the shrinkage in the principal plan of the part. It is shown that the profile of temperature quickly becomes uniform in the thickness of the part after the phase of ejection and that the kinetics of shrinkage is controlled by the kinetics of cooling for the PS as for PP. One shows the influence of certain processing parameters on the evolution of the shrinkage, in particular, the dependence of the shrinkage according to the temperature of the mould and the packing pressure. Finally one notes the presence of an anisotropy of shrinkage in the case of PP.

Osmotic shrinkage and reswelling of giant vesicles composed of dioleoylphosphatidylglycerol and cholesterol

The osmotic shrinkage of giant unilamellar dioleoylphosphatidylglycerol (DOPG) vesicles in a hypertonic osmotic solution is investigated. The volume reduction for given membrane area leads to a vesiculation of the bilayer into the interior of the giant. The size of the daughter vesicles that appear inside the giant is uniform and an increasing function of the cholesterol content, but independent of the osmotic gradient applied. The radius of the daughter vesicles increases from 0.2 μm to 3.0 μm when the cholesterol content is changed from 0 to 40%. It is argued that the size of the daughter vesicles is regulated by the membrane persistence length, which is an exponential function of the mean bending modulus. From the kinetics of shrinkage it follows that approximately 14% of the daughter vesicles remain attached to the mother giant. This is in reasonable agreement with osmotic swelling experiments which show that approximately 11% of the daughter vesicles is available for area expansion.

Analysis of Mechanisms of Spark-Plasma Sintering

Spark-Plasma Sintering (SPS) involves rapid heating of powder by electric current with simultaneous application of external pressure. Numerous experimental investigations point to the ability of SPS to render highly-dense powder products with the potential of grain size retention. The latter ability is of significance for the consolidation of nano-powder materials where the grain growth is one of the major problems. A model for spark-plasma sintering taking into consideration various mechanisms of material transport is developed. The results of modeling agree satisfactorily with the experimental data in terms of SPS shrinkage kinetics.

Kinetic Modeling of LTCC Shrinkage: Effect of Alumina Content

We studied sintering of LTCC-type glass matrix composites (GMCs) consisting of small glass and alumina particles of equal size. Primarily, crystals act as rigid inclusions, decelerating the densification rate. In later stages, they also dissolve, partially increasing the viscosity. Release of alumina finally induces crystallization of alumosilicates, which enables post-firing stability. To study both effects, two model GMCs were prepared: an α-Al2O3 + barium alumoborosilicate glass (BABS)–GMC, which shows neither significant dissolution nor crystallization, and an α-Al2O3 + calcium alumoborosilicate glass (CABS)–GMC, which dissolves readily and promotes crystallization. The kinetics of shrinkage for both GMC were modeled by utilizing Frenkel theory for the early stage and Mackenzie–Shuttleworth theory for the late stage, assuming that sintering is superimposed by the weighted contributions of triparticle glass–crystal clusters, their random occurrence (ideal mixing), and a shrinkage rate controlled by the GMC effective viscosity. In agreement with modeling, the experimental results showed that the shrinkage rate of BABS–GMC decreases progressively for crystal volume fractions Φ &amp;gt; 0.15. The attainable shrinkage is reduced by up to 8% for Φ = 0.45. For the CABS–GMC with Φ = 0.25, a reduction of Φ to 0.20 was evident due to partial α-Al2O3 dissolution. This effect was found able to increase the sintering temperature by ~50–60 K.

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30–50°C, relative humidity: 0–80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.

Studies of the mechanism and kinetics of a porous body compaction in intensive electric sintering. Report 3. Prediction of the shrinkage kinetics and comparison with experiment

A theoretical model and software have been developed for predicting the shrinkage of compacts from metal powders both at the stage of cold pressing, and in the process of their intensive electric sintering. The model of the shrinkage has been supplemented with equations of heat transfer in electrical heating and has been formulated as the initial boundary value problem that allows for special features of the real technological process. The input parameters of the model are compression pressure and the current strength as well as electrophysical and mechanical properties of the compressed material. The parametric numerical analysis of the model and its comparison with the experimental data has been made. Based on the results of the study, a simplified model has been proposed that makes possible engineering calculations and the optimal choice of technological parameters of the electric sintering.

Cathode edge displacement by voiding coupled with grain boundary grooving in bamboo like metallic interconnects by surface drift-diffusion under the capillary and electromigration forces

The kinetics of cathode edge shrinkage and displacement (drift) coupled strongly with the grain boundary (GB) grooving is investigated using the novel mathematical model developed by Ogurtani, in sandwich type thin film bamboo lines. The computer simulations are performed under the constant current (CC) and the switch-over constant voltage (SOCV) operations. The cathode drift velocity and the cathode failure time show the existence of two distinct phases, depending upon the normalized electron wind intensity parameter χ; the capillary ( χ ⩽ 0.01) and the electromigration (EM) dominating regimes ( χ > 0.01), having current exponent n, equal to 0 and 1, respectively. Analysis of various experimental data on the cathode drift velocity results a consistent value for the surface drift-diffusion coefficient, 1.0 × 10 - 5 exp ( - 1.00 eV/ kT ) m 2 s - 1 , for copper interconnects exposed to some contaminations during the processing and testing stages. This is found to be an excellent agreement with the experimental values reported in the literature after applying the proper 1/ kT correction on the apparent activation enthalpy associated with Nernst–Einstein mobility relationship. The complete cathode failure time (CCFT) due to the cathode area shrinkage by voiding is also formulated by inverse scaling and normalization procedures, which show exactly the same capillary and EM dominating regimes. This formula can be used to predict very accurate CCFT for metallic lines with bamboo-like, near-bamboo, and even with polycrystalline structures by proper calculation of the cathode-edge path length (CEPL) parameter, in terms of the actual line width, the thickness and the grain size.

Densification Behaviour of Nano-Size CeO&lt;sub&gt;2&lt;/sub&gt;

A nano-sized CeO2 powder was synthesized by a modified glycine nitrate process (MGNP). The synthesized powder was characterized by X-ray diffraction (XRD), the Brunauer Emmett Teller (BET) method, and transmission electron microscopy (TEM). The lattice parameter and crystallite size were determined by the Rietveld refinement of X-ray diffraction patterns. The shrinkage kinetics of the green body was continuously monitored in air and in oxygen atmospheres using a high temperature dilatometer up to 1500°C. During the high temperature sintering in air a redox reaction occurred (Ce4+ was partially reduced to Ce3+, and oxygen gas was released). The redox reaction influenced the sintering behaviour of CeO2, resulting in a decrease in density. On the basis of shrinkage kinetics data in oxygen atmosphere a master sintering curve for CeO2 was constructed. Using the concept of the master sintering curve, the densification behaviour in oxygen atmosphere was successfully predicted from early to final stages of sintering. During sintering of CeO2 at lower temperature in air atmosphere a significant contribution of the surface diffusion was observed.

Microstructural development and evolution of liquid phase sintered Cr-Cu composites

Features of the microstructure formation of Cr-Cu composites under impregnation followed by liquid phase sintering of reduced and electrolytic chromium powders at 1200?C in a vacuum of (2-4)?10-3Pa have been studied. The refractory component particle size distribution in the microstructure of samples with reduced chromium sintered for 60 min is shown to obey a normal logarithmic law; with distribution parameters being sensitive to the volume fraction of the refractory particles. The calculated values of the dihedral angle are close to the value of one of the modes in the experimental dihedral angle distribution for the microstructure of electrolytic chromium based samples (115?). The interfacial and interparticle surface energies ratio ?sl/?ss&gt;0.5 is shown to correspond to theory for the Crs-Cul system in equilibrium, which indicates the presence of skeleton structure elements in the course of composition formation under liquid phase sintering (including the case of excess liquid phase). Experimentally determined interparticle and interfacial surface areas, solid particle contiguity and continuity are discussed in terms of concurrent diffusion-controlled particle coarsening (in Lifshitz, Slyozov and Wagner theory) and particle coalescence (in German?s model). The kinetics of shrinkage for the composites with 50...55 % solid-phase volume-fractions at heating and isothermal sintering in a vacuum at a temperature of 1200?C in terms of linearly viscous rheological theory are discussed.

The effect of strain-path reversal on cavitation during hot torsion of Ti-6Al-4V

The effect of strain-path reversal on cavitation behavior during the hot torsion testing of an alpha/beta titanium alloy, Ti-6Al-4V, with a colony-alpha microstructure was established. Optical microscopy was used to measure cavitation parameters such as cavity size, density, and area fraction. It was observed that when the torsion direction is reversed, the cavitation process is reversed as well: i.e., cavity shrinkage takes place. The experimental observations were interpreted in the context of previous models developed for the densification of porous bodies. For this purpose, the models were modified to treat the effect of colony orientation on the local stress state and the accommodation of the externally imposed strain, both of which affect the rate of densification/cavity shrinkage. A modified version of the AFRL PM-consolidation model was shown to provide reasonable estimates of the shrinkage kinetics. An alternate description of cavity shrinkage during reversed torsion, analogous to prior descriptions of cavity growth, was also developed. It was concluded that the absolute magnitude of the cavity shrinkage rate is smaller than its counterpart during growth because the local stress ratio is lower during reversed straining compared to that during forward straining.