Control of rheology of water-borne paints using associative thickeners

Water-borne decorative topcoats generally show inferior leveling and open time compared to solvent-based paints. Basically, this behavior is caused by the divergent viscosity–solid content relationship for dispersions and emulsions and by the relatively high evaporation rate of water. Employment of associative thickeners may improve leveling and open time of latex paints only if they introduce a substantial amount of ‘network viscosity,' characterized by a short relaxation time and little dependence on solid content. This network viscosity enables one to formulate a paint with sufficient high-shear viscosity at a particle-packing density far below the value where low-shear viscosity starts to diverge. Addition of an associative thickener not only affects rheology, but also the interaction between latex particles: Associative HEUR thickeners may induce undesired phase separation by strong bridging between the latex particles. The influence of HEUR thickeners on latex particle interaction has been studied by turbidity measurements. The experimental results could qualitatively be interpreted very well by two-particle interaction potentials computed using the Self-Consistent-Field theory of Scheutjens and Fleer. It is demonstrated how viscosity, created by the addition of an associative thickener to a highly concentrated latex, can be split up into a polymer network viscosity and a contribution to (relative) dispersion viscosity. According to these analyses, reduction of the molecular weight of tri-block HEUR thickeners yields an increase of the favorable network viscosity and a reduction of the unfavorable dispersion viscosity. However, reduction of the molecular weight of the HEUR thickener is limited by the introduction of undesired phase separation (bridging flocculation) below a certain molecular weight.     

Rheological properties of hydrophobic ethoxylated urethane (HEUR) in the presence of methylated β-cyclodextrin

Hydrophobic ethoxylated urethane (HEUR) is one type of hydrophobically modified polyethylene glycol, which could form a transient network structure in aqueous solution via intermolecular hydrophobic associations. The viscosity of HEUR solution is dependent on two parameters; namely the effective elastic chain density and the relaxation time. In the present study, different amounts of methylated β-cyclodextrin (m-βCD) were added into aqueous HEUR solutions, and significant reduction in the solution viscosity was observed when the m-βCD concentration exceeds a critical value. Oscillatory shear indicated that the plateau modulus does not decrease appreciably with the addition of m-βCD, however a large reduction in relaxation time was observed. At low m-βCD concentrations, addition of m-βCD does not decouple the hydrophobic association. The ring-shaped hydrophobic inner cores of m-βCD interact with the hydrophobic moiety of looping HEUR chains, leading to a reduction in the relaxation time, which weakens the transient network. However, at higher m-βCD concentrations, bridging chains are decoupled, thereby disrupting the transient network and produce smaller isolated aggregates.     

Synthesis and rheological properties of an associative star polymer in aqueous solutions

Rheological properties of aqueous solutions and hydrogels formed by an amphiphilic star block copolymer, poly(acrylic acid)-block-polystyrene (PAA₅₄-b-PS₆)₄, were investigated as a function of the polymer concentration (C_p), temperature, and added salt concentration. The water-soluble polymer synthesised by atom transfer radical polymerization (ATRP) was found to form hydrogels at room temperature at polymer concentrations, C_p, over 22 g/L due to the interpolymer hydrophobic association of the PS blocks. Increasing C_p leads to stronger elastic networks at room temperature that show a gel-to-solution transition with increasing temperature. Increase of ionic strength decreases the moduli compared with the pure hydrogel but did not affect the gel-sol transition temperature significantly. Small-angle X-ray experiments showed two distinct scattering correlation peaks for samples above the gelling C_p, which indicates the aggregates formed due to hydrophobic association. Upon heating the intensity of the scattering correlation peaks was found to decrease indicating the loss of the network structure due to thermal motion.     

Hyperbranched poly(methyl methacrylate)s prepared by miniemulsion polymerization and their (non)-Newtonian flow behaviors

Miniemulsion polymerization is most suitable for the targeted synthesis of vinyl copolymers than the conventional emulsion polymerization, because in miniemulsion polymerization each monomer nanodroplet is a nanoreactor, and the monomers in each droplet are in situ converted to the corresponding polymers. Soluble and hyperbranched poly(methyl methacrylate)s (PMMA) were prepared with quantitative monomer conversion and without gelation by the miniemulsion copolymerization with di- and tri-acrylate and mediated with 1-dodecyl thiol (DDT). DDT acted both as a gelation prohibitor and as a reactive cosurfactant. The PMMAs with varied “X” or “Ж” shaped branches, depending on the di- and tri-functional acrylate used as the branching agent, are characterized and interpreted in terms of the repeating units per part, parts and branches per macromolecule, average molecular weight, latex particle size and size distribution. Effects of topology changes of the branched PMMAs on the rheological behaviors are observed for the first time: from Newtonian flow for the densely branched PMMAs to the non-Newtonian flow with pronounced shear thickening for the PMMA samples with high-molecular-weight and longer parts.     

Why is fresh self-compacting concrete shear thickening?

The rheological properties of fresh concrete are mostly described by means of the Bingham model. For self-compacting concrete, the Bingham model is applicable in a lot of cases, but some authors report that the rheological behaviour is non-linear. The apparent viscosity increases with increasing shear rate and the SCC shows shear thickening behaviour. Shear thickening becomes important in operations occurring at high shear rates, like mixing and pumping. In these cases, shear thickening should not be forgotten in order to avoid breaking of the mixer, pump or pipes.This paper will describe two possible theories for shear thickening behaviour of SCC, based on results published in the rheology literature. The first theory consists of the formation of so-called (hydro-)clusters, which are temporary assemblies of small particles. These clusters start being formed from a certain shear stress on: the critical shear stress. They cause the viscosity to increase with increasing shear rate. A second theory is based on grain inertia, where a part of the shearing force is transmitted through direct momentum transfer between solid particles. Results on cement pastes prove that the grain inertia theory is not the main cause of shear thickening in self-compacting concrete. The influence of several parameters on the shear thickening behaviour of SCC can be well explained by means of the cluster theory.     

Study of the rheology of aqueous radiation curable polyurethane dispersions modified with associative thickeners

The chemistry of radiation curable polyurethane dispersions is outlined with an emphasis on the microstructure of the aqueous polymer dispersion and the possible interactions with associative thickeners. The steady-shear flow was studied for two model dispersions prepared from the same unsaturated polyurethane but showing significantly different particle size distributions. A hydrophobically modified ethoxylated urethane (HEUR) associative thickener with a linear structure was incorporated at different amounts to the dispersions with varying particle volume fractions. The steady-state viscosity at 25 and 10 °C was always reached quickly after instant flow rate changes so that no significant thixotropic effects were reported within the experimental timescale. Without thickener, the flow curves of the two model dispersions exhibited a Newtonian behavior except at the highest volume fractions where shear thinning became apparent. The maximum packing values determined from the Krieger–Dougherty relationship were essentially the same for the two systems. In the presence of thickener, the flow curves were characterized by a Newtonian plateau followed by a marked shear thinning region even at low particle volume fractions. This behavior typically suggests the formation of a physical network between polyurethane particles and thickener molecules partly adsorbed onto the polymer surface. The zero-shear viscosity of the two dispersions was compared with respect to: (i) particle volume fraction and (ii) particle surface area at different HEUR concentrations. At a given volume fraction, the particle size affects the viscosity of thickened models. As a corollary, a relationship is found between the particles size and the level of thickener required to reach a target viscosity. This study offers practically relevant data in terms of application conditions and provides a better insight into the thickening protocol.     

Shear-induced nonisothermal crystallization of low-density polyethylene

The effect of melt memory on the shear-induced nonisothermal crystallization of a low-density polyethylene melt is analyzed with a shear DTA instrument. The melt state responsible for the saturation of shear-induced isothermal crystallization was identified previously as the steady state in steady shear flows and the strain applied to the melt was identified as the controlling factor for that saturation. Here it is shown that the same strain that saturates the isothermal crystallization also saturates the shear-induced nonisothermal crystallization. Regardless of the cooling rate, the nonisothermal crystallization, starting from the same sheared melt temperature, saturates at the same strain. The contribution of the melt memory effect to the overall nucleation density is estimated, and it is concluded that the majority of nuclei results from the thermal-induced crystallization.     

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.     

Shear Thickening in Dilute Polymer Solutions: Transient Analysis

Transient shear properties of dilute polymer solutions are investigated in the shear rate regions where shear thickening can occur. Comparison of transient rheological and optical data with the two coupled Maxwell modes model offers insight into the physical mechanisms that give rise to this anomalous behavior. Specifically, shear thickening occurs due to the deterioration of the size and anisotropy of structures deformed at lower shear rates. Coincident theoretical and experimental discontinuities in the transient profiles of the dichroic orientation angle appear to confirm the prior statement.     

Steady state flow of cement suspensions: A micromechanical state of the art

Fresh cementitious pastes can be viewed as suspensions of particles of many different sizes (from several tens of nm to 100 μm) in a continuous fluid phase. This broad poly-dispersity implies that various interactions such as surface forces (or colloidal interactions), Brownian forces, hydrodynamic forces or various contact forces between particles interplay. Depending on the volume fraction of the particles in the mixture, the use of admixtures or the magnitude of either the applied stress or strain rate, one or several of these interactions dominate. Our objective here is not to quantitatively predict rheology of cement pastes but rather to understand and classify the situations where, depending on composition and processing, one or other of the physical phenomena will control the macroscopic behavior. The result of this analysis is a conceptual diagram of predominant interactions in flowing cementitious suspensions under simple shear, as a function of shear rate and solid fraction.     

Shear-induced orientation in the crystallization of an isotactic polypropylene nanocomposite

Isothermal crystallization of isotactic polypropylene (iPP)/organic montmorillonite (OMMT) binary composite under shear field was investigated by in situ polarized optical microscopy, rheometry and transmission electron microscopy. When shear strain was small, shear flow could enhance the crystallization of iPP, and the crystallizing entity was spherulitic in iPP/OMMT composite in which the OMMT content was below the percolation threshold. With shear strain increasing, the orientation extent became stronger and cylindrites and strings of spherulites appeared in these samples. However, for iPP/OMMT composite with OMMT content higher than the percolation threshold, when the shear strain was not big enough to destroy the fillers network in the matrix, the crystallization of iPP was similar with that of the un-sheared sample. When shear strain was large enough, the fillers network was destroyed and clay layers were aligned along the flow direction. There formed oriented crystals including cylindrites and strings of spherulites, which were much smaller in size than those formed in the previous case, because the aligned clay layers acted as heterogeneous nucleation agents to promote crystallization of iPP.     

Non-linear shear deformation of hydrophobically modified polyelectrolyte systems

The shear imposed oscillation technique was employed to probe the shear-induced structural changes of 3 wt% hydrophobically modified polyelectrolyte solutions that possess shear-thinning and shear-thickening behavior. The shear-thickening behavior is related to the transformation of predominantly intra-molecular to inter-molecular associations. On the other hand, the shear-thinning behavior under moderate shear deformation is caused by the re-organization of the transient network structure. Under high shear deformation, the shear-thinning behavior is solely caused by the shear-induced effect that increases the chain-end exit rate, which reduces the mechanically active chains and lifetime of the hydrophobe in the micellar junctions.     

Shear-induced crystallization of poly(phenylene sulfide)

The crystalline morphology of poly(phenylene sulfide) (PPS) isothermally crystallized from the melt under shear has been observed by polarized optical microscope (POM) equipped with a CSS450 hot-stage. The shish–kebab-like fibrillar crystal structure is formed at a higher shear rate or for a longer shear time, which is ascribed to the tight aggregation of numerous oriented nuclei in the direction of shear. The crystallization induction time of PPS decreases with the shear time, indicating that the shear accelerates the formation of stable crystal nuclei. Under shear, the increase of spherulite growth rate results from highly oriented chains. The melting behavior of shear-induced crystallized PPS performed by differential scanning calorimetry (DSC) shows multiple melting peaks. The lower melting peak corresponds to melting of imperfect crystal, and the degree of crystal perfection decreases as the shear rate increases. The higher melting peak is related to the orientation of molecular chains. These oriented molecular chains form the orientation nuclei which have higher thermal stability than the kebab-like lamellae that are developed later. A new model based on the above observation has been proposed to explain the mechanism of shish–kebab-like fibrillar crystal formation under shear flow.     

Shear-induced crystallization in a blend of isotactic polypropylene and high density polyethylene

Shish-kebab morphologies were observed with relatively low shear rate and low temperature in the phase-separated isotactic polypropylene (iPP) and high density polyethylene (HDPE) blend. Both components are crystallizable polymers. In our experiments, relatively low shear rates and low temperatures were used, so that the entangled network chains cannot be broken up or disentangled, and the shish nuclei must be formed from oriented and stretched network chains instead of a bundle of pulled-out chains. The effects of shear rate, shear time and temperature on the formation and morphology of shish-kebabs were studied by in situ optical microscopy and shear hot stage under various thermal and shear histories. Optical microscopic measurement showed that the length of iPP cylindrites is much longer than the dimension of phase domains, which implies that iPP cylindrites grow through both iPP and HDPE phase domains. An unexpected ‘core-shell’ structure was observed in the melting procedure, which could be explained by the difference of crystallinity between ‘core’ and ‘shell’. It is most important that two kinds of shish-kebabs, the interface morphology and transcrystallites were observed by scanning electron microscopy (SEM). SEM observation also revealed that the width of iPP shish is about 1-2 μm and the width of HDPE shish is about 100 nm. The difference in the shish width probably resulted from the lower molecular weight, higher polydispersity, less inter-chain interaction force, and faster nucleation and growth rate of HDPE relative to the iPP chains.     

A NOVEL FOULING MONITOR

A tapered tube device is described which allows the influence of a range of shear stresses on the formation of a wall fouling layer to be examined in the same experiment. The device is demonstrated for fouling from reconstituted skimmed milk over a range of surface shear stresses and temperatures. The shear stress required to maintain a clean surface, the critical shear stress, is shown to a function of temperature, with an activation energy of 150 ± 10Kj/mol. A mechanism for the early stages of fouling is proposed, based on a simple application of turbulent burst theory, which explains the form of the experimental results. The fouling from reconstituted skimmed milk is shown to be a two-stage process. Suggestions for the modelling of such processes are given. The further development of the tapered tube device is outlined.     

Integration approach of the Couette inverse problem of powder type self-compacting concrete in a wide-gap concentric cylinder rheometer: Part II. Influence of mineral additions and chemical admixtures on the shear thickening flow behaviour

The influence of mineral additions and chemical admixtures on the shear thickening flow behaviour of powder type self-compacting concrete (SCC) is studied by means of a wide-gap concentric cylinder rheometer. The Couette inverse problem is treated by means of the integration method in order to derive the flow curve τ(γ?) from the torque measurements.According to the experimental results, the shear thickening effect is found to be strongly influenced by the addition of the chemical admixture (a polycarboxylate ether based superplasticizer), whereas mineral additions were found to modify the intensity of shear thickening. The limestone, quartzite and fly ash addition used in this research project, respectively increase, unalter and decrease the shear thickening intensity. The powder volume and the available amount of free water proved to have a major impact on the viscosity of the powder type SCC mixes. Increasing the powder volume or decreasing the amount of free water results in an increased viscosity of the SCC mix.     

Shear-induced orientation in polymer/clay dispersions via in situ X-ray scattering

We report in situ X-ray scattering measurements of shear-induced orientation in polymer-clay dispersions. Two different organically modified clays, montmorillonite and fluorohectorite, are dispersed in a low molecular weight, viscous polymer melt, facilitating studies at room temperature. Orientation measurements are performed in the flow-gradient plane, allowing characterization of both the average degree and direction of particle orientation during shear. In all cases, the orientation angle is finite, indicating systematic misalignment of the particle long axes relative to the flow direction. In concentrated fluorohectorite and montmorillonite dispersions, anisotropy and orientation angle are roughly independent of shear rate, and negligible relaxation is observed upon flow cessation. Conversely, a lower concentration montmorillonite sample exhibits orientation that is more responsive to shear flow, and partially relaxes upon flow cessation. In this sample, the orientation behavior is interpreted in light of rotational diffusion of the clay particles. This same sample exhibits oscillatory structural dynamics upon shear flow reversal, attributed to tumbling rotations of the disk-like clay particles in shear. Large-amplitude oscillatory shear is similarly demonstrated to be capable of inducing significant particle orientation; the degree of orientation is principally determined by the applied strain amplitude. Complementary measurements of rheological properties exhibit many characteristics commonly reported in polymer-clay nanocomposites. Based on the structural measurements reported here, the rheological phenomena are interpreted to arise from a combination of flow-induced particle orientation and rate- and time-dependent destruction or reformation of particle networks.     

Grafting of polyelectrolytes onto polyacrylamide by reactive processing

Low‐molecular‐weight high‐charge‐density cationic poly diallydimethyl ammonium chloride (polyDADMAC) was grafted onto nonionic polyacrylamide (PAM) using organic peroxide initiators in the molten state carried out in a batch mixer. The graft copolymer can be used as a high performance flocculant. Glycerol was selected as a plasticizer. The grafting reaction was characterized in terms of composition, temperature, degree of grafting, and grafting efficiency. It was found that free radicals on polymer chains were induced by the decomposition of the initiator. Grafting was produced by free radical recombination termination. The degree of grafting increases with an increase of the polyDADMAC/PAM feed ratio. However, the grafting efficiency was lower than 10 wt % in this highly viscous polymer melt system. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1412–1416, 1999     

Shear-induced fractal morphology of immiscible reactive polymer blends

The origin of shear-induced morphology of two-component immiscible reactive polymer blends is studied by the example of grafting and crosslinking multilayer systems of statistic terpolymer of ethylene, butyl acrylate, and maleic anhydride and statistic copolymers including polyamide and acid groups terminated by acid and/or amine groups. It is found that in contrast to the non-reactive system, the reactive polymer blends display pronounced hydrodynamic instabilities followed by the formation of branched fingers. The observed morphologies are shown to evolve towards the fractal structures. Their fractal dimensions depend on the type of chemical interactions between the blend components resulting either in grafted or crosslinked interfaces. It is shown that the obtained morphologies resemble the Laplacian growth patterns. A simple model of the interface chemical modifications is discussed to explain a physical origin of the observed shear-induced finger instability.