Rheological properties of aqueous polymer dispersions

Aqueous dispersion of nearly monodisperse charged polymer spheres of 250 nm diameter have been investigated in steady and oszillatory shear flow. Polydisperse latices were included for comparison. By using different types of rotational rheometers and a capillary viscometer viscosity measurements could be performed over a shear stress range from 10^-3 Pa to 10⁵ Pa. At shear stresses below 10^-2 Pa the viscosity is independent of the shear rate but strongly increases with qrowing volume fraction ?. In the stress range of 0.1 to 10 Pa a transition to a lower viscosity level is observed. For ??0.2 this transition is negligible and the dispersion behaves Newtonian. The viscosity decrease (shear-thinning behaviour) becomes significant at higher volume fractions and reaches a magnitude of more than 3 decades for ? = 0.50. Polydispersity increases the viscosity level at small stresses. The latter is also increased by growing particle charge (higher pH-value) but reduced by the addition of NaCl (screening of the charge). For volume fractions ? ?0.4 shear-thickening is found up to a viscosity maximum. The maqnitude of shear-thickening can be reduced by addition of NaOH (increase of particle charge) and broadening of the particle size distribution. The concentrated dispersions behave viscoelastic. They show an instantanious elastic deformation at the onset of shear creep tests and exhibit recoverable strains after unloading. In oscillatory shear the storage modulus approaches a constant value with decreasing angular frequency indicating the formation of a three-dimensional lattice structure.     

Rheological behavior of waterborne polyurethane/starch aqueous dispersions during cure

Isothermal rheological behaviors of waterborne polyurethane (WPU)/starch aqueous dispersions during cure were investigated with a small-amplitude oscillatory shear flow experiment to evaluate their crosslinked structure and to predict their mechanical properties, for the first time. An abrupt increase in the elastic storage modulus (G′), the viscous loss modulus (G″), the complex dynamic viscosity (η^*) and the loss tangent (tan δ) was observed during the curing process of the dispersions, as a result of the formation of a fractal polymer gel. The gel point (t_gel) was determined from the intersection in tan δ vs curing time for different constant shear frequencies, where tan δ was frequency independent and all curves crossed over, indicating the validity of the Winter–Chambon criterion for the complex system. The values of the power law exponent (n) and the gel strength (S_g) at the gel point indicated that with an increase of starch content the crosslinked WPU/starch gels underwent a transition from weak fractal to strong elastic ones. Moreover, the WPU/starch composite sheets, obtained from the aqueous dispersions with relatively high S_g values, also exhibited the increased tensile strength (σ_b) and Young's modulus (E). Their structure–mechanical properties relationship and the phase transitions of dispersed starch–dual-phase continuity–starch matrix were revealed. This work confirmed that the rheological characters could be used to predict the mechanical properties of the WPU materials blended with natural polymer.     

UV-nanoparticles: Photopolymerized polymer colloids from aqueous dispersions of acrylated oligomers

Polymer colloids are usually prepared using thermally activated polymerization. Photo-induced techniques for the preparation of polymerized or crosslinked colloids have seldom been reported in the literature and to our knowledge none has found real application in industry. Here, we introduce a new approach to form insoluble crosslinked colloidal particles by in situ photopolymerization starting from aqueous dispersions of auto-dispersible acrylated oligomers originally developed for water-based UV-coating applications. Of particular interest is the relatively high rate at which particle polymerization takes place even in highly opaque conditions e.g. with particle loadings of more than 20 w/w% and particle sizes ranging from 40 up to 500 nm. In this paper, the preparation process is demonstrated with an immersion-type batch photoreactor and the properties of this new family of organic colloids are highlighted. Furthermore, we report the preparation of submicron sized containers by loading a hydrophobic compound (i.e. n-hexadecane) within a water-dispersible urethane acrylate shell followed by solidification of the container wall using radical photopolymerization with a suitable photoinitiator. The thermal and colloidal characteristics of these novel capsules are discussed in some detail and the potential application as nanoscaled phase-change material is highlighted.     

Alkaline hydrolysis of aqueous polymer dispersions,particularly vinyl acetate copolymers

The influence of comonomer species and concentration on the ease of alkaline hydrolysis of vinyl acetate copolymers in the aqueous dispersion form is reported. The comonomers studied include higher vinyl esters, acrylic esters, a fumaric diester, and ethylene. The significance of the emulsion polymerization formulation has also been considered. The rate of hydrolysis is reduced with increasing proportions of comonomers and with increasing length and branching of the alkyl side chain originating from such comonomers. Branched long-chain tertiary vinyl esters reduce not only the rate but also the extent of hydrolysis, being resistant to hydrolysis themselves and also protecting part of the more susceptible vinyl acetate; inhomogeneous copolymers, specially prepared, were less resistant than the more homogeneous copolymer of the same overall composition. The inclusion of quite small amounts, such as 1% by weight, of acid comonomers has a relatively large effect, increasing ease of hydrolysis. For comparison, the behavior of higher vinyl ester homopolymers and methyl methacrylate copolymers with acrylic esters is included. It is concluded that the major factors influencing ease of hydrolysis are steric and other environmental effects arising from the copolymer microstructure.     

Rheological study of dispersions prepared with modified soybean protein isolates

Structural modifications of modified soy protein isolates (SPI) were distinguished by rheological behavior. SPI were prepared by acidic (pH 2.5) and thermal-acidic treatment without (pH 1.6) and with neutralization (pH 8.0). Dynamic properties of dispersions were determined through the variation of storage and loss moduli with frequency, and loss tangent behavior was analyzed. Changes in viscoelastic parameters with protein concentration (10–12% wt/vol) and time of heating (15–60 min) were also determined. Flow properties of dispersions were estimated through apparent viscosity and flow and consistency index measurements. Rheological behavior of dispersions was compared with those found by experiment with commercial mayonnaise, mustard, and salad dressing. The analysis of rheological parameters showed that thermally treated isolates formed dispersions with high elastic modulus and consistency index with a structure mainly stabilized by hydrophobic interactions, although no gelation process after cooling was observed. From the rheological point of view, it was deduced that thermally treated isolates could be used as ingredients in the formulation of salad dressings. The alkaline sample would be more versatile because, depending on protein concentration and thermal treatment, the consistency of its dispersions was like that in salad dressing, or similar to those of mustard and mayonnaise.     

A rheological study of concentrated aqueous nanotube dispersions

The rheological behaviour of aqueously dispersed oxidised nanotubes has been studied at concentrations at which the nanotubes interacted with each other. The dispersed nanotubes represented a high aspect ratio system with a ratio of 80. Dynamic and steady shear tests were applied to the dispersions using a cone and plate rheometer. The system was found to behave as a reversibly flocculated dispersion. The structure of the dispersions was highly strain-sensitive with the linear viscoelastic region (LVR) extending to strains of 1%. The moduli within the LVR were independent of frequency and scaled with concentration by a power law. Under steady shear the dispersions rapidly shear thinned up to a Peclet number of 1 to 10. At higher Peclet numbers the shear thinning behaviour followed the Ostwald–de Waele power law. The dispersions were thixotropic and recovered their structure, and hence viscosity, upon standing.     

Preparation and characterization of acrylic polymer nanocomposite films obtained from aqueous dispersions

A functionalized fumed silica was dispersed in water using a nonionic surfactant, yielding a stable nanodispersion. This was blended with an aqueous acrylic polymer dispersion to produce hybrid nanocomposite films. The silica particles were shown to be well dispersed in the polymer matrix, with little agglomeration. Further evidence of good compatibility between the silica and acrylic polymer was given by the improved thermal stability of the nanocomposite compared with the pristine polymer. The nanocomposite films exhibited significantly lower dirt pick‐up behavior, which seems to be associated to the nanoroughness of the composite film surface observed in AFM analysis. This decreases the contact area between film and micrometric dirt particles. Surface tension and hardness do not seem to be significantly different in the composite and noncomposite materials. This approach may provide a strategy to obtain hybrid coatings with self‐cleaning properties, taking advantage of the relatively low cost, and large availability of fumed silica. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Rheological properties of sodium montmorillonite dispersions

The rheological behaviour of dispersions of montmorillonite in water is highly pH- and salt-dependent. This is exemplified for 4% (wt/wt) dispersions of montmorillonite from Wyoming and its homoionic sodium form in water, 0.01 M NaCl and 0.1 M NaCl solutions, at different pH and at temperatures from 20°C to 60°C. Generally, the shear stress (for instance, at shear rates of about 100 s⁻¹) decreases sharply in an acidic medium to a minimum around pH 6 (pH 7.5 for the homoionic sodium form) and then increases again very strongly. Thixotropic and antithixotropic behaviour depend on pH, salt concentration and temperature.The presence of Ca²⁺ ions (molar ratio Na/Ca about 6.5/1) in the “pristine” montmorillonite affects the type of flow very sensitively. Ca²⁺ ions convert diffuse ionic layers into quasicrystalline structures with a central layer of gegen ions. Attractive potentials are then created in the contact regions between the particles, even in diluted salt solutions. Formation of band-like structures (as in the “Bändermodell” of A. Weiss) is promoted.Further, results are reported on the influence of particle size (fractions < 0.06 μm to 2 μm) on the flow behaviour at different concentrations of homoionic sodium montmorillonites from Amory and Cameron.     

Rheological properties of alumina-polyacrylate gel dispersions

Rheological properties of aqueous alumina-polyacrylate gel dispersions have been investigated with a controlled stress rheometer as functions of both cross-linked polyacrylate and alumina concentrations at pH 13. These studies were conducted to understand the role of alumina-polyacrylate interactions in building the rheology of complex liquids that contain surfactants, sodium hypochlorite, alkali, and inorganic salts. Viscosity and viscoelasticity of polyacrylate gel dispersions can be enhanced by the addition of colloidal alumina. The trends in viscosity and viscoelasticity are explained on the basis of changes in the conformation of polyacrylate and repulsive interactions of negatively charged polyacrylate and alumina particles.     

紫外光固化水性聚氨酯丙烯酸酯分散液流变行为

用甲苯二异氰酸酯、聚己二酸新戊二醇酯多元醇、二羟甲基丙酸(DMPA)和甲基丙烯酸羟乙酯合成了紫外光固化聚氨酯丙烯酸酯(PUA)水分散液。该预聚物组成通过FTIR和^1H-NMR进行了表征,并对其水分散液的流变行为进行了研究。结果表明,随预聚物中DMPA质量分数的增加,分散液由牛顿流体转变为非牛顿流体。当DMPA质量分数为6．68％时,PUA水分散液的粒径为40～70nm,ζ电位为-66．00mV,黏流活化能为59．08J／mol。     

Rheological properties of fumed silica filled Bis-GMA dispersions

The rheological properties of Bis-GMA dispersions filled with fumed silica were investigated to optimize the manufacturing process and mechanical properties by using a Rheometrics Mechanical Spectrometer (RMS). Steady and dynamic measurements on the RMS were carried out to obtain shear viscosity and dynamic mechanical properties. The effect of several factors on the rheological properties of fumed silica dispersions was also examined. The factors were a concentration of a silane coupling agent (γ-MPS), the methods of surface treatment of fillers, silica content, diluent concentration, shear rate, and operating temperatur. From these studies, it was observed that shear viscosity showed an asymptotic phenomenon at a higher concentration than a uniform multi-layer coverage concentration of γ-MPS. The silane coupling agent had a significant role in the reduction of tan δ, resulting from a decrease of loss modulus, while fillers decreased tan δ by increasing the storage modulus. In cases where the silica content and diluent concentration increased simultaneously, the Barcol hardness of Bis-GMA/silica composites was increased, but there was no change in the viscosity of dispersions and diametral tensile strength of those composites.     

PUA复合水分散体的发展

主要从聚合方法和多重复合改性两方面较为系统地介绍了丙烯酸聚氨酯复合水分散体的研究进展。比较了互穿网络法、种子乳液法和原位乳液聚合法等的特点和进展,对多重复合改性如有机硅、纳米材料等的进展作了简介。     

Rheological study of compositional heterogeneity in an associative commercial polymer solution

The compositional heterogeneity of a concentrated aqueous solution of a commercial hydrophobically associating polysaccharide and its precursor has been investigated, using optical and rheometrical characterization. Optical observations proved that the non-associating precursor solution was rather homogeneous, whereas significant compositional heterogeneity was present in the associating one, containing soft polymeric particles with a size of order 100 μm. Linear and non-linear rheological measurements were carried out on the centrifuged and non-centrifuged associating polymer solution in order to study the rheological influence of these objects and their nature. The results tend to prove that the soft polymeric particles are mainly composed of polymer chains bearing the longest hydrophobic groups.     

Interfacial rheology of uniform polymer dispersions

The effects of high oscillatory shear on the viscosity of a model polymer dispersion of solid particles was investigated. A dispersion of glass beads in poly(dimethyl siloxane) (PDMS) is uniform and free of any agglomeration. When sheared at low stress levels, the dispersion remains uniform. At high stresses under oscillatory shear, two processes can take place. One is spatial ordering (threading) of the spherical beads that does not occur in steady shear. The other is molecular interfacial slip due to high interfacial stress in samples containing fluoro-silane treated glass beads, leading to a lower suspension viscosity than that of samples containing bare non-treated beads.     

水性聚氨酯分散液的研究进展

评述了水性聚氨酯分散液的作用和重要性,综述了水性聚氨酯分散液的研究进展,对水性聚氨酯分散液的合成制备方法进行了归纳总结,认为在制备水性聚氨酯过程中,同时利用异氰酸酯基和不饱和双键的反应活性,形成互穿聚合物网络体系是目前合成制备水性聚氨酯分散液的主要思路和方法,最后,对水性聚氨酯分散液的应用发展趋势进行了展望。     

Association thickening of polymer latex dispersions

Low levels of addition of hydrophobically modified water-soluble polymers to dispersions of colloidal polymer latices dramatically alter their rheological behaviour as measured over a wide range of simple shear rates. This measured rheological behaviour for a variety of thickener-latex systems demonstrates that the thickening mechanism is not merely that of thickening the continuous phase, water. Clearly an association between the principal components of the dispersion, the polymer latex and the thickening polymer, takes place. Additionally, in coatings formulations, surfactants and cosolvents are present and these contribute to and alter the mechanism of the thickening process. A variety of mechanisms can be proposed which influence the observed thickening. A set of polymer latices and associative thickeners have been used to experimentally investigate the sensitivities of the interactions and their effect on the rheology of the system.     

Dielectric dispersions in a commercial acrylonitrile-butadiene-styrene polymer

Observations of dielectric constant and loss from ?120°C to +140°C over the frequency range 30 Hz to 3MHz are presented for a low rubber content acrylonitrile-butadiene-styrene (ABS) polymer of commercial origin. Seven dielectric dispersions are distinguished in the tan δ contour map and constant-frequency plots together with evidence for an eighth at the lowest temperatures. Possible sources for each dispersion are proposed in relation to the miscibility and morphology of the individual phases present in this multicomponent polymeric material.     

原位一步法制备温敏改性纳米二氧化硅

采用原位一步法合成了温敏改性聚 N-异丙基丙烯酰胺(PNIPAAm)/SiO2纳米复合物. 通过接触角对温敏改性的纳米SiO2进行了条件优化：反应温度73℃,反应时间6 h,单体用量与KH-570摩尔比为1:1,引发剂用量为单体用量与KH-570总质量的2.5%。通过TEM、FT-IR、XPS等手段对温敏性纳米SiO2的组成和结构进行了表征。同时,这一有机-无机纳米复合物仍然保持PNIPAAm 的温度响应性, 最低临界溶解温度(LCST)与纯PNIPAAm相似。