Carbopol Dispersions Research Articles

Imaging of the microstructure of Carbopol dispersions and correlation with their macroelasticity: A micro- and macrorheological study

We developed a new data analysis strategy, the so-called micro-rheo-mapping technique, based on multiparticle tracking experiments to obtain an accurate and direct visualization of the microstructure of commercial acrylate thickeners of Carbopol-type with high (Ultrez 10), intermediate (ETD 2020), and low (ETD 2050) degree of crosslinking. At low polymer concentration, aggregates made of several primary Carbopol particles are formed with an average diameter of 43 ± 11, 56 ± 14, and 10 ± 2.5 μm for Ultrez 10, ETD 2020, and ETD 2050, respectively. For ETD 2050, the least crosslinked thickener, the shell of dangling polymer chains covering the aggregate surface is thicker than for ETD 2020 and Ultrez 10. At technically relevant polymer concentrations, our results indicate, for all three thickeners, that the microstructure is highly heterogeneous with regions of different crosslink densities. One region inaccessible for tracer particles corresponding to a mixture of polydisperse aggregates and individual primary particles with a core mesh size less than 200 nm and a second, diluted enough to be accessible and which exhibits both elastic and viscous characteristics. The study of the impact of pH, polymer concentration, and crosslink density on these local structural and viscoelastic heterogeneities as well as macrorheological properties allowed us to establish a correlation between microstructure and macroelasticity. In particular, we found that the bulk shear modulus strongly depends on the fraction of inaccessible areas, making this microscopic parameter most relevant for describing the macroelasticity of Carbopol gels, whereas the local elasticity of the interstitial regions is of minor importance.

Effect of D-Mannitol on the Microstructure and Rheology of Non-Aqueous Carbopol Microgels.

D-mannitol is a common polyol that is used as additive in pharmaceutical and personal care product formulations. We investigated its effect on the microstructure and rheology of novel non-aqueous Carbopol dispersions employing traditional and time-resolved rheological analysis. We considered two types of sample, (i) fresh (i.e., mannitol completely dissolved in solution) and aged (i.e., visible in crystalline form). The analysis of the intracycle rheological transitions that were observed for different samples revealed that, when completely dissolved in solution, mannitol does not alter the rheological behaviour of the Carbopol dispersions. This highlights that the chemical similarity of the additive with the molecules of the surrounding solvent allows preserving the swollen dimension and interparticle interactions of the Carbopol molecules. Conversely, when crystals are present, a hierarchical structure forms, consisting of a small dispersed phase (Carbopol) agglomerated around a big dispersed phase (crystals). In keeping with this microstructural picture, as the concentration of Carbopol reduces, the local dynamics of the crystals gradually start to control the integrity of the microstructure. Rheologically, this results in a higher elasticity of the suspensions at infinitesimal deformations, but a fragile yielding process at intermediate strains.

Signatures of Overaging in an Aqueous Dispersion of Carbopol.

In this work, we study the effect of the deformation field on the physical aging behavior of an aqueous Carbopol dispersion. It is composed of soft swollen particles of gel that get deformed and acquire a polygonal shape, with flat interfaces rendering the dispersion a soft solid-like consistency as filled volume fraction approaches unity. It has been proposed that owing to release of stored elastic energy in the deformed particles, Carbopol dispersion undergoes microstructural evolution that is reminiscent of physical aging in soft glassy materials. We observe that application of moderate magnitude of oscillatory strain to Carbopol dispersion slows down its relaxation dynamics, thereby showing characteristics of overaging. On the other hand, the sufficiently high magnitude of strain makes the relaxation dynamics faster, causing rejuvenation. We also solve the soft glassy rheology model, which, when subjected to the same flow field, corroborates with experimental observations on the Carbopol dispersion. This behavior, therefore, suggests that in a system of jammed soft particles of Carbopol, the particles occupying shallow energy wells upon application of moderate strain field adjust themselves in such a manner that they predominantly occupy the deeper energy wells leading to observe the overaging dynamics.

Rheology of graphene oxide suspended in yield stress fluid

• Suspensions of graphene oxide (GO) in Carbopol are also elastic yield stress fluids. • Addition of GO nanosheets in Carbopol decreases viscosity and elasticity. • Oxidation level of GO affects the rheological behavior. • Higher level of interaction of the GO/water system oxidized for 96 h. This work investigates the rheology of graphene oxide (GO) suspension on a yield stress model fluid. The effects caused by the variation of the GO concentration in the suspension and by the amount of oxygenated groups pendant present on the nanosheets surface, on the rheology of the suspensions were evaluated. The dispersant used was an aqueous solution of Carbopol® Ultrez 10. This is a transparent, nontoxic elastic yield stress fluid. GO nanosheets were produced from synthesis of graphite oxide (GrO) by modified Hummers method, and the nanosheets were characterized by XRD, Raman, FTIR, TEM and AFM techniques. The rheological behavior of these suspensions was characterized by oscillatory and steady-state flow experiments. The GO structure characterization shows that oxygenated functional groups were incorporated in its graphite surface in different levels after using two distinct times of oxidation. GO oxidized for 96 h (GO 96 h) showed greater interplanar distance and also presented few layers when compared with GO oxidized for 2 h (GO 2 h). The GO exfoliation process directly into the aqueous dispersion of Carbopol® showed to be an effective method. As for the pure Carbopol solutions, the GO suspensions were well modeled by the Hershel-Bulkley equation. The increase of GO concentration in the suspensions impairs the level of fluid structure and leads to a decrease in viscosity, yield stress, and elasticity. When compared, the GO 96 h promoted a lower decrease in viscosity, yield stress and elasticity than the GO 2 h suspension. For the suspension with a higher concentration of GO 96 h, it was observed the appearance of hysteresis at low shear rates. These results show that small changes in the GO nanosheets surface can significantly influence the rheological responses of a non-Newtonian fluid.

Signatures of physical aging and thixotropy in aqueous dispersion of Carbopol

In this work, we investigate signatures of physical aging in an aqueous dispersion of Carbopol that shows yield stress and weak enhancement in elastic modulus as a function of time. We observe that the creep curves, as well as strain recovery, show a significant dependence on waiting time elapsed since shear melting. The corrected strain, which is the strain in excess of the recovered strain, has been observed to show time–waiting time superposition in the effective time domain, wherein time is normalized by time dependent relaxation time that shows a power-law dependence. The corresponding power law exponent, which is close to unity in a limit of small stresses, decreases with stress and tends to zero as stress approaches the yield stress. For a range of stresses, the material shows time–stress superposition suggesting the shape of the evolving relaxation time spectrum to be independent of the time as well as the stress. This work, therefore, suggests the presence of physical aging in an aqueous dispersion of Carbopol even though the elastic modulus shows only a weak enhancement. We also discuss the Andrade type of creep behavior in aqueous Carbopol dispersion.

Gelation kinetics of non-aqueous Carbopol dispersions

The time evolution of the gelation process of non-aqueous Carbopol dispersions in PEG and glycerol was studied experimentally by tracking the evolving viscoelastic properties of the material by means of Fourier Transform Mechanical Spectroscopy (FTMS). A structural conversion degree related to the storage modulus was defined and a kinetic expression to describe its evolution was developed. UV–vis spectroscopy was employed to confirm the mechanism of gelation. It was found that an increase in both the concentration of glycerol and the gelation temperature increased the gelation rate. These trends seem to indicate a diffusion-controlled mechanism of the process, in which the solvent molecules penetrate the cross-linked structure of Carbopol particles, causing their uncoiling and subsequent swelling. Lastly, the kinetic relation found for the structural conversion degree was used to obtain conversion maps, which can be employed as a first guidance to design new continuous gelation formulation processes.

Quasi-static bubble in a yield stress fluid: elasto-plastic model

The influence of the yield stress of Carbopol® gel dispersions on the behaviour of quasi-static bubbles was investigated. Many fluids, from many different industrial fields, have yield stress behaviour. Most of them contain gas bubbles. To study bubble behaviour in such suspensions, a transparent model fluid (dispersion of Carbopol® in water) was used. The experimental device allowed to quasi-statically increase bubble internal pressure with small pressure step to reach a maximum target internal pressure and the pressure setpoint was inverted to return to the initial pressure. Hysterical behaviour of the bubbles was highlighted as they did not regain their initial shape because of yield stress. We show that the rheological behaviour is related to the internal pressure, bubble geometry and yield stress in quasi-static conditions. A modification of the Laplace law depending on the yield stress of the fluid and bubble sphericity was proposed and validated.

Scaling and mesostructure of Carbopol dispersions

Rheological measurements were performed on aqueous dispersions of two commercial crosslinked polymer microgels, Carbopol Ultrez 10 and Carbopol ETD 2050, prepared over a wide range of concentration and pH. For all concentrations studied, both the yield stress and the elastic modulus initially increased dramatically with pH and displayed broad peaks at intermediate pH. This is consistent with the onset of jamming of the Carbopol particles due to a rapid increase in particle size caused by osmotic swelling in the presence of NaOH. Scaling of both yield stress and elasticity with concentration was observed only at higher concentrations, which we believe indicates a change from a percolated structure at low volume fractions to a space filling network of compressed particles at high volume fractions. This model is supported by confocal microscopy of fluorescently dyed Carbopol dispersions.

Use of Carbopol 980 and carboxymethyl cellulose polymers as rheology modifiers of sodium-bentonite water dispersions

The effects of polymer addition on the rheological parameters of sodium bentonite water dispersions at ambient conditions were studied using high molecular mass carboxymethylcellulose (CMC) and Carbopol 980. Adsorption isotherms using the batch equilibrium technique of the polymers onto the bentonite particles were Langmuir isotherms of the L1 type, indicating monolayer adsorption of the polymers onto the surface of the bentonite particles. The aqueous dispersions of 3% and 4% sodium bentonite exhibited Herschel–Bulkley rheological behavior. Addition of CMC up to 1.5% by mass to the 3% sodium bentonite dispersions decreased the yield stress and the flow consistency index because of the steric effects caused by the adsorption of the polymer. This state was then followed by a plateau of the yield stress and a considerable increase of the flow consistency index, indicating that after a particular polymer concentration, further addition merely increased the liquid viscosity of the mixture. The flow behavior index was not affected by CMC addition. Addition of Carbopol 980 to the 3% and 4% sodium bentonite dispersions up to 0.15% by mass again firstly decreased the yield stress and the flow consistency index, then increased the yield stress and the flow consistency index with increasing polymer concentration. The high shear viscosity of bentonite–Carbopol dispersions showed also a minimum followed by a drastic increase. The flow behavior index was not affected significantly by the polymer addition.

Transient Shear Banding in a Simple Yield Stress Fluid

We report a large set of experimental data which demonstrates that a simple yield stress fluid, i.e., which does not present aging or thixotropy, exhibits transient shear banding before reaching a steady state characterized by a homogeneous, linear velocity profile. The duration of the transient regime decreases as a power law with the applied shear rate γ. This power-law behavior, observed here in carbopol dispersions, does not depend on the gap width and on the boundary conditions for a given sample preparation. For γ≲0.1 s(-1), heterogeneous flows could be observed for as long as 10(5) s. These local dynamics account for the ultraslow stress relaxation observed at low shear rates.

Use of Carbopol resin for carp pituitary administration improves the fertilization percentage of northern pike (Esox lucius Linnaeus) eggs in commercial hatcheries

The efficiency of northern pike (Esox lucius Linnaeus) fry production via hormonal treatment of wintered broodstock is, in general, relatively low due to low egg fertilization percentage. It has been experimentally demonstrated that administration of acetone-dried carp pituitary extract in a slow-release vehicle of aqueous dispersion of Carbopol 971 P resin (CP) resulted in a higher mean fertilization percentage, possibly because the gradual hormone action could optimally control ovarian follicle maturation and ovulation. This new method of hormonal induction of ovulation was tested in a large-scale hatchery production of northern pike fry. In the present study, data on fertilization percentage collected between 2002 and 2006 at two prominent hatcheries in Hungary were analyzed. Administration of acetone-dried carp pituitary in a 2.5% CP vehicle resulted in higher fertilization (76.4 ± 7.7%; mean ± SD) compared to the saline vehicle group (59.3 ± 10.8%). The vehicle used did not affect the ovulation ratio. According to these results from large-scale production, the new method could be suitable to have wide application for induced breeding of northern pike.

Cyclodextrin/carbopol micro-scale interpenetrating networks (ms-IPNs) for drug delivery

Cross-linking of hydroxypropyl-β-cyclodextrin (HPβCD) with ethyleneglycol diglycidylether (EGDE) in carbopol dispersions enabled the synthesis of cyclodextrin hydrogels with domains of interpenetrating acrylic microgels (micro-scale-IPNs) in a single step under mild conditions. As carbopol proportion increased, the hardness and compressibility of the ms-IPNs decreased, but their bioadhesion force and pH-responsiveness rose. Control HPβCD hydrogel and ms-IPNs were loaded with estradiol and ketoconazole by immersion in drug suspensions, some of which were autoclaved to enhance (up to a 50%) drug/cyclodextrin affinity. ms-IPNs prepared with 0.8% or 1.0% carbopol showed the highest loading due to their greater swelling degree and, consequently, mesh size. The total loading of the ms-IPNs greatly exceeded (up to 200-fold) the amount dissolved in their aqueous phase, which highlights the main role of drug complexation with the cross-linked cyclodextrins. The affinity of the drug for HPβCD sustained the release for several days; the rate being also dependent on carbopol content and on pH of the medium. Therefore, an adequate design of the HPβCD/carbopol ms-IPNs provides a single material with tunable mechanical properties, in which the complexation ability of cyclodextrins is combined with the bioadhesive and pH-responsive properties of carbopol. The ms-IPNs are potentially useful as vehicles of relatively hydrophobic substances.

Carbopol gels: Elastoviscoplastic and slippery glasses made of individual swollen sponges: Meso- and macroscopic properties, constitutive equations and scaling laws

This paper gives details of new data on neutralized Carbopol 940 dispersions. Appropriate techniques have been used to characterize the physical properties of the bulk gel and inter-phase slip at the wall. Previously published data are analysed and used wherever possible. Terminology and measurement difficulties are also addressed. The structure of Carbopol dispersions can be described in terms of polydisperse glasses made of individual swollen hydrophylic elastic sponges. They display elastoplasticity, and significant dissipation both below and above their yield stress. Hardly any creep was observed over 10 months of experiments. Scaling laws are proposed for Carbopol mechanical properties as a function of concentration. Carbopol behaviour varies with the value of concentration, and in particular when comparing percolation concentration with close-packing concentration. Constitutive equations for bulk shear stresses and for friction slip at the wall can be deduced from contact mechanics which fit rheometry data and scaling laws. Herschel–Bulkley constitutive equations may be used by ignoring elasticity, normal shear stresses and transients.

Fourier transform infrared spectroscopy for the analysis of neutralizer-carbomer and surfactant-carbomer interactions in aqueous, hydroalcoholic, and anhydrous gel formulations

The objective of the present study is to evaluate the polymer-surfactant and polymer-neutralizer interactions in topical aqueous, anhydrous, and hydroalcoholic gel formulations using Fourier transform infrared (FTIR) spectroscopy. The gels were prepared by dispersing Carbomer (Carbopol 980) in water and ethanol for aqueous and anhydrous systems, respectively. Glycerol and propylene glycol were also added to ensure that the compositions of gels closely resembled those used in typical topical gel formulations. Comparisons of the spectra of Carbopol dispersions in aqueous, anhydrous, and hydroalcoholic systems, performed for the first time, show Carbopol-neutralizer and Carbopol-surfactant interactions vary depending on the nature of the solvents used for gel formation. Analysis of the spectra of aqueous gel formulations indicates significant presence of ionized carboxyl groups only at higher pH (approximately 8.0). Drying of the aqueous gels causes a shift in the carbonyl stretch band toward higher energy, suggesting changes in polymer-neutralizer interaction. Anhydrous gels exhibit 2 different carbonyl stretch bands: the one at approximately 1653 cm(-1) is related to the carboxyl group that is hydrogen bonded and is akin to hydrous gels; the second one at approximately 1717 cm(-1) is indicative of free carbonyl groups. The carbonyl bands of dried gels appear at different energy levels than the solvated gels. This shift resulting from solvent evaporation, reported for the first time, indicates changes in hydrogen bond characteristics. The results show that FTIR can be a good technique compared with other more time-consuming means of analysis for topical formulations.

New measurements of the flow-curves for Carbopol dispersions without slip artefacts

The thickening properties of many commercial thickeners are difficult to measure because of wall slip artefacts. Here we report a series of experiments on a typical thickener where these artefacts have been successfully eliminated. As a result, complete, steady-state flow-curves of aqueous Carbopol 980 (the toxicologically preferred version of the older and more well-known Carbopol 940) dispersions are reported for a range of concentrations of 0.045–1.0 wt%. The vane-and-basket flow geometry was used to avoid slip problems at low shear stress, with the geometry housed in a TA AR1000-N controlled-stress rheometer, whilst a Haake RV2 viscometer with an SV2P and MV2P concentric-cylinder geometries were used at higher shear rates. The flow-curves obtained show a smooth but steep transition from a very high Newtonian viscosity at low shear stress to a much lower viscosity at high shear stress. No real yield stresses were detected, but the higher shear rate results can be fitted to the Herschel-Bulkley model, which assumes an apparent yield stress. The various model parameters are displayed as a function of Carbopol concentration.

The use of dilute solution viscometry to characterize the network properties of carbopol microgels

We used dilute solution viscometry, specifically, measurements of intrinsic viscosity and overlap concentration to characterize two members of the carbopol family, carbopols 940 and 941. These measured quantities were then used to calculate the swollen-to-dry volume ratios for both resins over a range of ionic strengths. The variation in this ratio is representative of the swelling equilibria of the micronetwork and is modeled using standard network theory with modifications for non-Gaussian chain statistics and for fixed charges on the chains. By fitting to experimental data, the cross-link density is determined as 1450 monomer units between cross-link sites for carbopol 940 and 3300 units for carbopol 941. The shear modulus of the microgel domain is then predicted from the swelling data and found to be an order of magnitude greater than the elastic modulus measured on concentrated carbopol dispersions. We argue that this discrepancy is due to a large number of defects which reduce the cycle rank of the network. Current microgel technology specifies that their rheological behavior can be predicted once the domain cross-link density is known. Simple dilute solution viscometry appears capable of providing this prerequisite.

Evaluation of carbopol 934 as a suspending agent for sulphaoimidine suspensions

The suitability of carbopol 934 as a suspending agent was investigated by continuous shear and creep analysis using a concentric cylinder air turbine viscometer. Apparent vistosities determined from the apices of flow curves at a constant shear rate of 87 s −1 varied little with temperature over the range of 10–50°C. Decrease in pH from 8 to 6 caused a reduction in apparent viscosity. Non-Newtonian, pseudoplastic-like flow curves with little hysteresis were obtained for 0.1–3% w/v neutralized carbopol 934 dispersions, the magnitude of their apparent static yield value being linearly related to the polymer concentration. Analysis of creep compliance curves for carbopol dispersions showed that the material was a viscoelastic liquid at 0.5 and 1% and a viscoelastic solid at 3%. Suspensions containing 10% w/v sulphadimidine and variable concentration of carbopol 934 gave qualitatively similar rheological results. Permanent suspensions after 6 months storage at room temperature were obtained for sulphadimidine suspensions using 0.3% or greater carbopol 934. Accelerated sedimentation tests on sulphadimidine—carbopol suspensions involving centrifugal techniques correlated well with the results of longterm sedimentation studies. Measurement of apparent static yield values and apparent viscosities at 87 s −1 of such suspensions were also found useful for predicting their longterm sedimentation appearance.

Laminar flow heat transfer

AbstractNumerical solutions of the equations of motion and energy for the heating of non‐Newtonian fluids in rectilinear, axisymmetric laminar flow in tubes of circular cross section are extended to the case of cooling at constant tube‐wall temperature. The fluid density, heat capacity, and thermal conductivity are assumed constant, but the flow properties are represented by the temperature‐dependent equation τ = m(Ṡ exp ΔH‡/RT)n. The results are presented as plots of average NNu vs. NGz, with flow and temperature‐effect indices as parameters. The mean deviation of experimental data for the cooling of aqueous CMC and carbopol dispersions from the numerical solutions is ±8%.