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Waxy crude oil emulsion has thixotropic properties at the temperature near gel point, which is a macro-mechanical characterization of the structure failure and recovery of waxy crude oil emulsion. In this paper, the thixotropic behaviors of waxy crude oil emulsion near gel point were studied using hysteresis loop formed by stress linear increase and decrease, as well as the structural recovery characteristics. The influence of the loading conditions and water content on the thixotropy of waxy crude oil emulsion were analyzed with hysteresis loop area. The concept of “structural recovery” was introduced to study the degree of structural recovery after different stewing, and influencing factors were taken into account. Results have shown that for waxy crude oil emulsion, the failure to fully restore of the structure after lysis is the cause of the formation of hysteresis loop, and the loading conditions will not affect the strength of thixotropy and the degree of structural recovery. Additionally, the dispersed phase droplets weaken the thixotropy and structure recovery characteristics of waxy crude oil emulsion, and the greater the water content, the weaker the thixotropy. The findings can help to better understand the safe and economic operation of waxy crude oil-water pipeline transportation.

The viscosity evolution for different temperatures was studied experimentally. A time-varying viscosity model was derived and the influence of the initial temperature on gel time was analyzed. The experimental results show that the viscosity of polymer grout increases exponentially with time. It can be divided into two phases. Before gelation, the viscosity variable quantity is very small. At the gel point, there is a sudden increase in viscosity. The initial viscosity and gel time decrease with the increasing initial temperature within a certain range, The study contributes to deepening understanding of the rheological properties of polymer grout, which can provide some references for polymer grouting construction.

In this paper, a novel ethylene-vinyl acetate copolymer (EVA)/amino-functionalized polymethylsilsesquioxane (PAMSQ) composite wax inhibitor (WI) was prepared through melt blending, and the rheological and wax depositing properties of the doped crude oil were studied. The EVA/PAMSQ composite can strongly crystallize with wax molecules and trigger the formation of compact wax flocs, which greatly improve the rheological properties of crude: compared to the pure EVA at the same dosage, the composite can further decrease the gel point and apparent viscosity (25 °C) by 14.6 °C and 92.0%, respectively. The wax deposition can also be inhibited by adding the composite: adding EVA inhibits the mass of the wax deposit at 24 h by 62.3%, and this number is suppressed by 77.9% after adding the EVA/PAMSQ composite, but the wax molecule diffusion is accelerated and the wax content in the wax deposit rises because of the improved crude oil rheological characteristics. Here, we develop the gel structure theory to consider the wax deposition process affected by the WI. Adding the WI contributes little to the wax precipitation temperature but decreases the gel structural strength of the oil a lot, and thus, the oil near the pipe wall can only form the wax deposit with a higher wax content to resist the pipe wall shearing. Therefore, the crude oil added EVA/PAMSQ composite possesses not only the lowest wax deposit rate and the highest aging rate but also the highest wax content in the wax deposit.

The rheology of aqueous solutions of Pluronic F127 is studied as a function of concentration, temperature, and shear rate. At sufficiently low temperatures, the solutions behave like Newtonian fluids; a simple empirical model is proposed for the viscosity as a function of temperature and concentration. The solutions undergo a transition to a gel at higher temperature, above which a complex rheological behavior is observed. In this regime, the solutions are viscoplastic with a yield stress that can be as large as hundreds of Pascal. We provide Herschel–Bulkley fits to the rheology for a range of temperatures above the gel point. At much higher temperatures, the rheology of the solutions becomes unsteady and difficult to characterize in terms of a steady-state flow curve.

The gel point is the lowest solids content at which a fibre suspension forms a continuously connected network and is related to the fibre aspect ratio. In this paper we firstly investigated the conditions required to accurately measure a gel point using sedimentation. We found that very heavily treated cellulose nanofibres can produce anomalous sedimentation data, due to the electrostatic repulsion from fibre surface charges dominating the gravity driven sedimentation. Screening the anionic surface charges by adding high levels of Na+ or Ca2+ ions reduced the electrostatic interactions between the fibres and allowed them to settle normally. Gel point measurement was then used to probe the development of nanofibre quality with increasing energy input for three different feedstocks: eucalypt kraft pulp, commercial microfibrillated cellulose and de-lignified, bleached spinifex pulp. By combining the data of the aspect ratio and average diameter, determined from SEM and TEM, we were able to compare the differences in feedstock processability. The aspect ratio of all three feedstocks increased with increasing homogenisation energy, showing that the fibre delamination dominated over fibre shortening. The slope of the aspect ratio versus energy consumption showed the ease of processing of each sample. The spinifex fibres had the fastest rate of aspect ratio increase and therefore were the most processable. Gel point is an excellent tool to track quality development of nanocellulose through processing and to compare the potential of different feedstocks for nanocellulose production.

Thermal-induced gelation and phase separation was investigated in PAN/DMSO/H2O semidilute solutions utilizing rheological and optical methods. The incipient precipitation temperature, where a distinct decrease in relative light transmittance occurred, was considered as the cloud point (CP). Phase separation mechanism of nucleation and growth was confirmed by various ramp rates experiments and viscoelastic evolution pattern. On the other hand, the validity of Chambon–Winter model was evaluated in these blends and used to measure the critical gel point (GP). It was found that in the accessible temperature regions CP was always higher than GP, and phase boundaries were constructed accordingly. What's more, the absolute complex viscosity η′ at a low frequency also showed transition points during temperature decreasing experiments, which mostly located between CPs and GPs. The gelation mechanisms with and without phase separation were proposed and the effect of the two transitions on the structures of the ternary blends were shown by SEM pictures.

The temperature and concentration dependences of the transmission spectrum of an aqueous solution of agar are investigated. Measurements taken in the heating and cooling modes show strong thermal hysteresis in the spectral properties. Analysis of the results suggests that the formation of a gel network occurs in a much narrower temperature range than its collapse. The study of the temperature dependence of transmittance indicates that the association bispirals by cooling the solution and responsible for the formation of network occurs at temperature about [Formula: see text]C. A breakdown of these associates by heating of the gel begins at [Formula: see text]C and covers a wide temperature range. The gel point [Formula: see text] and the gel melting point [Formula: see text] increase with increasing polymer concentration, but for [Formula: see text] this dependence is stronger. Growth of turbidity (light scattering) of gel by increasing of agar concentration is due to the increasing of number and size of gel associates.

Some high water-cut crude oils can flow in the temperature below the oil gel point, while oil particles may adhere to the pipe wall as paste; this process is known as 'wall sticking'. This can cause partial or even total blocking of the transportation pipe. Several experiments using a laboratory flow loop were conducted to study the wall sticking characteristics of high water-cut crude oils. The experimental results indicated that the predominant influencing factors of wall sticking included shear stress, water-cut and differences between gel point and wall temperature. The wall sticking rate and occurrence temperature decrease with the increase of water-cut and shear stress. The criterion for the wall sticking occurrence temperature (WSOT), and the regression formula of the wall sticking thickness for high water-cut crude oil were then established. Typical case studies indicated that the prediction results obtained from the WSOT criterion and the wall sticking thickness regression formula were in accordance with the measured values. The wall sticking rate and WSOT vary widely under different conditions and it is necessary to consider its non-uniformity in production.

The characterization of the thermo-gelation mechanism and properties of ethyl cellulose/canola oil oleogels was performed using rheology and thermal analysis. Thermal analysis detected no evidence for thermal transitions contributed to secondary conformational changes, suggesting a gelation mechanism that does not involve secondary ordered structure formation. Rheological analysis demonstrated a relationship between the polymer molecular weight and the final gel strength, the cross-over behavior as well as the gel point temperature. Increasing polymer molecular weight led to an increase in final gel strength, the modulus at cross-over, and the gel point temperature. Cooling/heating rates affect gel modulus only for the low molecular weight samples. A decrease in gel strength with increasing cooling rate was detected. The cross-over temperature was not affected by the cooling/heating rates. Cooling rate also affected the gelation setting time where slow cooling rates produced a stable gel faster.

A one dimensional dewatering model for a thickener consistent with time-dependent densification of aggregates within a sludge suspension has been developed in this paper. The effects of different densification rate parameters and different underflow solids fluxes on the predictions of the sludge rheological properties and thickener performance have been explored. Pre-shearing of aggregates (which leads to the aggregates densifying to some extent before entering the thickener consolidating bed) must occur if a large underflow solids flux is required in a densified thickener. This increases the suspension gel point, and hence the solids volume fraction at the top of the bed. The solids volume fraction at the top of the bed will however be the initial undensified gel point in the case of a small underflow solids flux, where no pre-shearing of aggregates needs to occur. The effects of densification and/or pre-shearing on the sludge rheological properties and thickener performance have been predicted. For a densified thickener, an algorithm has been developed to determine the maximum permitted underflow solids flux, qmax, assuming any possible solids volume fraction at the top of the bed and the maximum underflow solids flux, qmu, constraining to the initial undensified gel point at the top of the bed. Regarding the aggregate densification rate parameter, as this parameter was increased, the sludge rheological properties evolved significantly during thickening and thickener performance was substantially enhanced. Moreover, the total solids residence time required to achieve the desired underflow solids volume fraction was significantly reduced both for a higher densification rate parameter and in the presence of pre-shearing in a densified thickener.

Cellulose nanofibre aspect ratio controls the properties of sheets made from nanofibres and processing conditions, but aspect ratio is very difficult to measure. In this paper, aspect ratio was estimated from the gel point of a cellulose nanofibre suspension, the solids concentration at which the transition from a dilute to a semi-dilute suspension occurs. Four batches of cellulose nanofibres were tested. Two were produced from softwood fibres using ball milling. Commercially produced microfibrillated cellulose material was also used, both in as supplied form and after removal of the larger fibres by filtering. The average diameter measured from SEM images of fibres ranged from 33 to 73 nm. One sample was too heavily treated and an average dimension could not be measured. The gel-point was measured both from the height of a layer of cellulose nanofibres sedimented from a dilute suspension or from the lowest solids concentration at which a yield stress could be measured using a vane rheometer. The two methods were closely in agreement for all samples. Aspect ratio was then calculated using either the effective medium (EMT) or crowding number (CN) theories. Aspect ratio calculated with an assumed fibre density of 1,500 kg/m3, using the CN theory ranged from 155 to 60. Use of the EMT theory reduced the calculated aspect ratio by between 11 and 23 %. Reducing the assumed density in suspension from 1,500 to 1,166 kg/m3 reduced the calculated aspect ratio by 12–14 %. The heavily treated sample had by far the lowest aspect ratio.

An experimental rheometric investigation is performed to assess the utility of various gelation point determination methodologies. Controlled shear stress and controlled shear rate methods are developed to investigate gelation processes in waxy oils. The nonoscillatory rheological method exhibits high reproducibility for model wax oils as well as for real crude oils. In addition, the nonoscillatory methods are in good agreement with standard oscillatory gel point determination methods. The effect of imposed shear stress, imposed shear rate, and cooling rate on gelation point is investigated with two types of model oils: 5 wt % macrocrystalline wax in Primol 352 and 5 wt % macrocrystalline wax in dodecane. In addition, the effect of wax content on gel temperature of macrocrystalline wax in dodecane is investigated. Moreover, the effect of additives (A3 and A4) on gelation point of a real crude oil UL-YS1 is investigated. Finally, the gelation temperatures of 5 wt % macrocrystalline wax in Primol 352, crude...

The presence of coarse particulate components in tailings streams poses significant challenges to the mineral processing industry. Suspensions with fast-settling coarse fractions make traditional rheological measurements tedious and inaccurate. These measurements are critical to the design and operation of mixing, dewatering, pumping and tailings equipment and operations. A methodology for predicting the gel point, close packing fraction and shear rheology of particulate slurries containing a binary mixture of fine and coarse material is presented. The model has validity subject to the constraint that the particle size of the coarse material is sufficiently distinct from that of the fine material. The model enables calculation of the gel point and close packing fraction of the mixture based on the gel points and close packing fractions of the pure fine and coarse materials in the suspension. The model predicts that the gel point (and close packing fraction) of the fine material increases with increasing coarse fraction until a maximum value is attained before decreasing to the gel point (or close packing fraction) of the coarse material. This relationship provides a subsequent basis for a yield stress model to accurately predict the yielding behaviour of binary mixtures of fine and coarse material. The model is useful in determining whether a sample exhibits yield stress behaviour through knowledge of the gel point and close packing fraction of the constitutive fine and coarse components. The model is exemplified using particulate mixtures containing coagulated aluminium oxide, calcium carbonate and coarse sand.

In the first chapter the definition of sol, gel , and sol-to-gel transition have been introduced; a main issue which still remains to be discussed is how to measure the gel point in the different systems. This is not a trivial aspect because only indirect measures are possible and no latent heat is released during the transition.

A silorane-based composite was compared against methacrylate-based composites in terms of shrinkage characteristics, thermal properties, gel point, and vitrification point. Shrinkage strain was measured using a laser triangulation method. Shrinkage stress was measured using a stress analyzer. Heat flow during photopolymerization was measured using photo-DSC. Statistical analysis was performed using one-way ANOVA and Tukey's test (p=0.05). Silorane exhibited significantly lower shrinkage strain than the methacrylate-based composites. It also presented the lowest stress values during light exposure, but the highest maximum stress rate after light exposure. It showed the highest heat flow rate, and it took the longest time to reach gel and vitrification points. Silorane demonstrated improved performance over the methacrylate-based composites with delayed gel and vitrification points as well as reduced shrinkage strain and stress. However, a high quantity of heat was liberated during the curing process, causing silorane to show significantly higher stress rate ( p

Wax deposition causes serious problems during oil production and transportation through pipelines. In this study, the influence of disulfide oil (DSO), ethylene vinyl acetate (EVA) copolymer as pour point depressant (PPD), SiO2 and clay nanoparticles on rheological properties of waxy oils are investigated. The viscoelastic modulus, viscosity, pour point, surface tension and yield stress of different waxy oil samples were measured. It was observed that the gel point, pour point, viscosity, gel strength, and yield stress decreased upon addition of these additives to waxy oil. It was found that rheological properties of oil samples improved by increasing SiO2 and clay nanoparticles up to 0.2 wt% in nanohybrid PPD (NPPD) and decreased at higher concentrations. Also, results indicated that SiO2 nanoparticles had better performance for wax prevention compared to clay, EVA, and DSO. Our measurements showed that yield stress decreased to 0.049 Pa by adding 0.12 wt% of SiO2 nanoparticle into NPPD whereas it was 54.3 Pa for blank oil. Also, pour point and surface tension of oil doped with SiO2 NPPD significantly reduced by increasing nanoparticle concentration. Moreover, it was found that DSO as a waste by product of gas refinery has a performance similar to EVA for wax prevention.

A semi-empirical spectrum model is proposed to describe the experimental data of the steady shear properties of a Shengli waxy crude oil near its gel point, where sophisticated structural effects become apparent due to the existence of waxy crystals in the crude oil. The model, consisting of a time spectrum, can well fit the steady shear viscosities of the waxy crude oil over the whole experimental shear rate region from 10−4 to 102 s−1. Two other experiments on complex fluids reported recently in the literature are also well described by this model demonstrating the applicability and accuracy of the model.

The homogeneity of PAN/DMSO/H2O spinning dopes with conventional PAN molecular weight and comparative high PAN concentration have been investigated using rheology method. The rheological results show that G′ and G″ of these solutions increase with increasing oscillation frequency. At lower temperature and higher H2O content, higher viscous modulus and elastic modulus are observed, too. The homogeneity of PAN/DMSO/H2O systems is reflected through the logG′-logG″ plots as well as gel point temperatures. The theoretical analysis indicates PAN/DMSO/H2O spinning solutions with conventional PAN molecular weight are close to homogeneous systems, which are very suitable for gel spinning.

Thermal-induced gelation for waterborne polyurethane dispersion has been studied rheologically under isothermal condition over a wide range of frequencies at different constant temperatures (55, 60, 65, and 70 °C). The elastic storage modulus, G‘, at a constant temperature in the vicinity of the gel point increases abruptly, and the magnitude of the elevation in G‘ was found to be temperature dependent. Similar behavior has been observed for both the viscous loss modulus, G‘‘, and the complex dynamic viscosity, η*. The gel point, tgel, was determined from the point of intersection in tan δ vs gelation time for different constant shear frequencies, where tan δ is frequency independent and all curves cross over, indicating the validity of the Winter−Chambon criterion. The value of tgel obtained from the coincidence of G‘ and G‘‘ was in excellent agreement with that obtained from tan δ vs t. At the gel point, G‘ and G‘‘ showed a power law with shear frequency, i.e., G‘ ∼ G‘‘ ∼ ωn with critical exponents n‘ a...

The evolution of native egg yolk undergoing a thermal-induced sol-gel transition was studied by using temperature controlled small amplitude oscillatory shear measurements. The critical gel point was determined according to Winter’s criterion: 1) from the measurements of storage and loss moduli as a function of heating time at different frequencies, and 2) from the exponents of the power law mechanical spectra obtained after cure experiments performed up to a maximum temperature (60–90 °C) followed by a sudden decrease in temperature up to 20 °C. Differential Scanning Calorimetry (DSC) was performed in order to investigate thermal transitions in egg yolk. Microstructure of gels was evaluated by Transmission and Scanning Electron Microscopy. The results obtained were discussed in terms of the processes involved in protein gelation: change in the protein system, aggregation of partially denaturated protein molecules and association of aggregates. As a result, an elastic gel network was always obtained. The influence of frequency, heating rate, solids concentration and maximum temperature of processing, was analysed. Most of the transformations found during thermal processing were found to be basically irreversible, even at the sol state and gel point. However, some reversible phenomena were detected during constant temperature processing depending on the maximum temperature performed.

A novel approach has been developed to measure in-situ chemical shrinkage of epoxy resins at the temperature of cure, during which the epoxy resins pass through liquid, rubbery and glassy states. A small sample of MY750/HY917/DY073 epoxy resin system, sealed in a thin-walled silicone bag, was suspended in a pot of silicone fluid and weighed independently of the silicone bath. The buoyancy of the sample was monitored as its density increases with respect to the constant density fluid during isothermal cures at three different temperatures. The relationship between the chemical shrinkage and degree of cure was deduced from a cure kinetics model for the resin. The good match of the results for the three different cure cycles suggests that chemical shrinkage is only a function of degree of cure regardless of time and temperature. A bi-linear relationship was fitted to the experimental data. The break point is at a degree of cure of about 28%, with corresponding chemical shrinkage of 3%. This point is linked to the gel point of the resin, which was measured as approximately 25% degree of cure in previous work. Total cure shrinkage of 6.9% for the MY750 resin system was obtained by extrapolating the results to a degree of cure of 100%. The method is sensitive, reliable and keeps the resin stress-free; therefore it should be applicable to a wide range of materials.

Thermally induced coupling of gelation and phase separation in polysaccharide aqueous solutions has a complex feature because of critical and tricritical phenomena, thermally induced hydrophobic interaction, and molecular-weight distribution of the polysaccharide. To elucidate the process, the criticality of a hydroxypropylmethylcellulose (HPMC) aqueous solution was assessed, and then dielectric relaxation and fluorescence intensity experiments were carried out. The diffusion coefficient of the solution with a weight fraction of HPMC being 0.06 could be extrapolated to zero at the cloud-point curve which showed the criticality of the solution. The fluorescence intensity increased at a temperature much lower than the cloud point and the gel point, especially for concentrated solutions, indicating the hydrophobic interaction as the driving force of the gelation coupled by the phase separation. Dielectric relaxation measurements by time-domain reflectometry revealed two characteristic relaxations of chain motions around 100 MHz and orientation of free water around 20 GHz , which is accompanied by a low-frequency tail reflecting hydration water.

Prediction of gel point conversion and network formation is of great importance in polycondensation during synthesis as well as processing. It enables one to estimate the safe conversions for reactor operation without gelation and the cycle time during processing, and plays an important role in controlling the molding parameters used for reinforced reaction injection molding (RRIM), reaction injection molding (RIM) and compression molding. Theories of gelation have been extensively published in the literature and supported by experimental data for various polycondensation systems. However, most such studies relate to unfilled systems. In this work, metal-filled polyurethanes have been synthesized in bulk by reacting toluene di-isocyanate with castor oil and its polyols possessing different hydroxyl values. Metallic aluminum powder (10–40% by weight) was dispersed thoroughly in castor oil and its polyols before reacting at different temperatures (30–60 °C) in a moisture-free, inert environment. The gel point conversions were measured experimentally and an empirical model from the experimental data has been developed to predict the gelation behavior. The proposed model could be of immense importance in the paints, adhesives and lacquers industries, which use castor oil in bulk. From these experiments it was concluded that: (i) fine metal powder gives a rise in viscosity; (ii) metal fillers not only restrict the molecular motion due to the increase in viscosity, but also lower the conversion; (iii) the vegetable oil and its polyols have a number of bulky groups, which also impart the delay tendency in gel time; (iv) there is a change in gelation dynamics at 50 °C – this is due to the change in reactivity of di-isocyanates; (v) the presence of metal filler does not initiate the intermolecular condensation; (vi) there is a gap between theoretical and experimental gel point owing to the unequal reactivity of the secondary alcohol position; (vii) there is an inverse relationship of gel time with the reaction temperature and hydroxyl value of polyols. An empirical model based on process parameters, i.e., hydroxyl value, temperature, shape factor and filler concentration, has been derived and found to be adequate for the metal-filled system. The correlation coefficient on the data is on the lower side in some cases because the following were not taken into account: (i) the first-order kinetics followed by the reaction in the second half while it is tending towards gelation; (ii) the error in observing the gel point viscosity; (iii) errors in assuming the spherical shape of aluminum metal powder; (iv) errors due to failure to maintain the constant speed in agitation. Copyright © 2003 John Wiley & Sons, Ltd.

High molecular weight paraffins are known to form gels of complex morphology at low temperatures due to the low solubility of these compounds in aromatic or naphthene-base oil solvents. The characteristics of these gels are strong functions of the shear and thermal histories of these samples. A model system of wax and oil was used to understand the gelation process of these mixtures. A significant depression in the gel point of a wax-oil sample was observed by either decreasing the cooling rate or increasing the steady shear stress. The wax-oil sample separates into two layers of different characteristics, a gel-like layer and a liquid-like layer, when sheared with a controlled-stress rheometer at high steady shear stresses and low cooling rates. The phase diagram of the model wax-oil system, obtained using a controlled-stress rheometer, was verified by analyzing the wax content of the incipient gel deposits formed on the wall of a flow loop. Based on the rheological measurements, a law has been suggested for the prediction of the wax content of the gel deposit on the laboratory flow loop walls. The wax content of the incipient gel formed on the wall of a field subsea pipeline was predicted to be much higher than that for the flow loop at similar operating conditions. This variation in the gel deposit characteristics is due to the significant differences in the cooling histories in the two cases.

Linear low-density polyethylene (LLDPE) was irradiated with electron beam radiation at room temperature and in the melt. It was found that the gel point, as measured by an insoluble fraction in boiling xylene, occurred at a slightly lower radiation dose for melt irradiated samples. The melt radiation-crosslinked samples showed different melt rheological properties from the room-temperature radiation-crosslinked samples, as exemplified by the stress relaxing more slowly in a step strain experiment (relaxation modulus) for the former at a given radiation dose. The relaxation modulus could be fitted to a power-law expression consisting of two parameters, which, when plotted against each other, resulted in a single master curve for both irradiation temperatures. The curve showed two distinct regions of differing slope, which intersect at a point argued to represent a pseudogel point.

Many thickening operations continue to be constrained by their ability to process consolidated underflows with high yield stress and associated rheological characteristics. The application of high molecular weight polymers (flocculants) is a prerequisite to maintain the operation integrity and dynamics of most gravity based solids/liquid separation processes. Such treatments are an effective means of consolidating solids and maximising clean water recovery. Unfortunately polymer/solids interaction at, and beyond, the gel point result in a significant increase the yield stress for a given solids content. BASF is developing a novel flocculant treatment (NFT) by which primary solid/liquid separation efficiency is maintained, and the subsequent consolidation process is enhanced, wherein higher underflow solids are associated with reduced yield stress. This paper will present application data to demonstrate the benefits associated with this NFT.

Effect of viscosity of an epoxy near or over its gel point on foaming structures was investigated. Rheological properties of the sample with a weight ratio 100/1 of epoxy/curing agent were measured by a simple shear experiment with sinusoidally varying shear. The magnitude of complex viscosity η* as a function of time increases rapidly after a definite time period from the time when the temperature is set to 90 °C and then increases slowly after η* reached about 1×106 Pa⋅s. The sample is found to gelate in the range of time t = 15.0∼15.6 min by the oscillatory measurement. The samples with a weight ratio 100/1/0.5 of epoxy/curing agent/blowing agent were foamed by putting them for 5 min in a furnace of 230 °C after various precuring conditions at 90 °C. There are roughly two sizes of bubbles, large bubbles (>100 μm) and small ones (≅30 μm), when the sample is precured for relatively short time before foaming. On the other hand, foams precured for a long time before foaming, large sizes of bubbles disappea...

Nature allows polymers to pass through the gel point with striking simplicity and elegance. However, what are the rheological properties at the gel point? Is the polymer at the gel point, the ‘critical gel’, at a distinct material state that has its own properties? It seems that these questions, until recently, have not been pursued. The questions might have been avoided since it was well known that the established rheological concepts fail altogether at the gel point: the concept of a viscosity is inapplicable since the viscosity diverges to infinity and the concept of an equilibrium modulus is useless since the modulus is still zero at the gel point. The gel point itself had remained mysterious.

In the production of pigment-coated paper or paperboard, the method and rate of drying of the coating will significantly influence the print quality of the finished product. Improper drying during initial st sages can cause binder migration that leads to its non-uniform concentration on the surface of the coating. Such effect causes print mottle, and is a primary reason for poor print quality. The location at which binder immobilization has occurred is called the Gel Point. An on-line fiber optic sensor system has been applied to continuously monitor the location of the Gel Point. Each sensor consists of a light source, an optical fiber for delivering the radiation, two optical fibers for diffuse and specular radiation monitoring and two detectors. The ratio R between diffuse and specular signals indicates coating conditions. A system description and experimental results are presented. The Gel Point measurement system has show that is provides a sensitive measurement of the gel point location, as it is affected by machine speed, coat weight, incoming substrate moisture levels and drying energy and can predict print mottle. This provides the paper maker with a completely independent method for monitoring the coating drying process.

The gel-setting and gel-melting temperatures of aqueous gelatin and agar solutions (0.5 wt% and 1 wt%) were measured by applying a steady state hot-wire method during pressurization up to 200 MPa. Pressurization induced the elevation of both the gel-setting and gel-melting temperatures in proportion to the pressure. The hot-wire method was found to be effective for assessing gelation under pressurized conditions.

Dynamic oscillatory experiments are used to monitor the gelation of the borosilicate systems prepared through the sol-gel process from metal alkoxides. The rheological experiments show that tan δ = G”/G’ is independent of frequency at the gel point in agreement with the results of others on organic gelling systems. The dynamic moduli at the gel point followed power-law behavior with respect to frequency. The power-law exponent is found to be ∼0.70. The apparent fractal dimension, dp, of the network cluster at the gel point is determined. The dF values for the samples ranged from 2.5 to 3.8 depending on the final structure of the evolved products at the gel point. The large values (dF > 3) exclude a simple geometric interpretation of the results. The effect of processing parameters, such as composition of reactants and temperature, on the resulting microstructures near the gel point is discussed.

A number of scales for sol fraction and irradiation dose have been tested in order to linearize gel-formation kinetics. The logarithm of sol fraction vs square root of irradiation dose has been discovered to be most suitable for the logarithmic-normal distribution. The plots remain reasonably linear up to about 70% gel fraction.

Moulding thermoset resin laminates under autoclave conditions removes the possibility of using resin sampling to determine the gelation point of the resin. The gelation point is critical, since it is then that pressure has to be applied to achieve optimum mechanical properties. This paper describes the use of automatic dielectrometry to monitor the process under autoclave conditions, giving an accurate prediction of the gel point. The need to ‘stage’ the prepreg system is eliminated, and optimum mechanical properties of the resulting laminate can be obtained.

The two constants in the equation log η = A + C′ Mn1/2 (η is the viscosity of molten alkyd and Mn the number average molecular weight) were determined at 110°C for two kinds of alkyd resin prepared with the same formulation but with different cooking schedules. It was found that the magnitude of the slope C′ was larger for the alkyd which was prepared by raising the reaction temperature directly up to 230°C, in comparison with that of the alkyd which was prepared by maintaining the temperature at 170°C for an hour and then raising it up to 230°C. Measurements of η and Mn were carried out until the gelation occurred. Both upward and downward breaks were observed in log η vs. Mn1/2 plot. Based on these viscometric data, the gel point mechanism was discussed. Disagreements between the molecular weight observed and that calculated from the Flory's theoretical equation became more remarkable as the esterification proceeded. This suggested that a large extent of intraesterification reaction is taking place in alkyd synthesis.

Tablets with a methylcellulose coating were found to exhibit lower dissolution profiles than those coated with a hydroxypropyl methylcellulose coating at 37 degrees, and the cause was investigated. The differences are attributed to thermal gelation of the methylcellulose at temperatures near 37 degrees, which creates a barrier to the dissolution process and essentially changes the dissolution mechanism. This mechanism is substantiated by the fact that at temperatures below the gel point and at increased agitation, the effect disappears. The retarded dissolution effect is not peculiar to the drug involved.

From fundamental and technological points of view RIM is a complex process.1 Fast reactions are required to form, in many applications, network materials and the reactions must by synchronised with the process so that the mould is filled with liquid reacting material, the material becoming solid at complete filling. The rates of reaction occurring between the two liquid reactant feeds which are mixed prior to mould filling must be well controlled, as must be the molecular growth process leading to the final, solid material. The important point in the polymerization is the gel point, which in terms of reactant flow defines the point of infinite viscosity.2,3