Stacy G. Bike

Development of a self-consolidating engineered cementitious composite employing electrosteric dispersion/stabilization

A self-consolidating engineered cementitious composite (ECC) reinforced with hydrophobic polyethylene fibers has been developed by combining micromechanics based design and rheological design, in a compatible manner. The previously developed micromechanics based design selects material ingredients for tensile ductility in the hardened state. The rheological design, which is the focus in this paper, modifies the material ingredients for self-consolidation behavior in the fresh state. For this purpose, the rheological design adopts a complementary electrosteric dispersion and stabilization technique to obtain cement pastes with desirable flow properties at constant particle concentrations dictated by the micromechanics based design. Such stabilization is realized by optimizing the dosages of strong polyelectrolyte and non-ionic polymer and by controlling the mixing procedure of the polymers. The fresh cement paste designed thereby leads to fresh mortar mix with desirable deformability, cohesiveness, and high consistency, and thus satisfies the self-consolidating performance of fresh ECC mix. In addition, ductile strain-hardening performance of the self-consolidating ECC is confirmed through uniaxial tensile test. This ductile composite with excellent fluidity can be broadly utilized for a variety of applications, e.g. in repair of deteriorated infrastructures requiring horizontal formworks, or in seismic-resistant structures with dense reinforcements and requiring high ductility.

Constitutive rheological control to develop a self-consolidating engineered cementitious composite reinforced with hydrophilic poly(vinyl alcohol) fibers

A self-consolidating engineered cementitious composite (ECC), which exhibits tensile strain-hardening behavior in the hardened state, while maintaining self-consolidating properties in the fresh state, has been developed by employing hydrophilic poly(vinyl alcohol) (PVA) fibers. The constitutive rheological design approach is adopted to separately control the aggregation between cement particles and sedimentation behavior with a combination of a strong polyelectrolyte and non-ionic polymer. This study suggests an effective formulation approach of fresh cementitious mix to maximize its fluidity without segregation, regardless of solids concentration employed. The resulting self-consolidating PVA-ECC exhibits tensile strain up to 5%. Besides, the methodology of constitutive rheological control can be extended to formulating other self-consolidating cementitious materials with various types of polymeric admixtures.

Measuring colloidal forces using evanescent wave scattering

The evanescent wave scattering technique total internal reflection microscopy has enabled the direct measurement of the mean potential energy of interaction between a Brownian particle and a flat surface. With a distance resolution of 1 nm and a force resolution of 10 fN, this technique has successfully measured a variety of colloidal forces. Recent measurements of van der Waals interactions have given rise to new theories for the effect of surface roughness on the interaction. In addition, recent measurements of depletion interactions have shown that energetic as well as entropic effects must be considered when computing the interaction potential.

Linear viscoelastic behavior of copper phthalocyanine dispersions used in printing inks

In this paper we describe the linear viscoelastic properties of copper phthalocyanine (CuPCN) dispersions that are used in the manufacturing of offset lithographic printing inks. Transmission electron microscopy shows that the primary pigment particles are rod-like and have sizes in the range of 10 to 300 nm. Steady shear measurements show that the dispersions are Newtonian at a pigment volume fraction of 0.073 and become increasingly shear thinning as the pigment volume fraction is increased. The strong shear-thinning nature of these dispersions can be attributed to the highly flocculated nature of the dispersions, which is due to interparticle attractions. The structural complexity of the dispersions also results in an unexpected linear viscoelastic response. While at low frequencies (0.1 and 1.0 Hz) the ex tent of the linear region decreases with increasing pigment concentration, at a higher frequency (10 Hz) the extent of the linear region increases with increasing pigment concentration. This increase in the linear region with increasing pigment concentration suggests that at higher frequencies the dispersion is less brittle, and that the rheological behavior is dominated by intra-aggregate associations. In addition, frequency sweeps show that the dispersions behave like a viscoelastic liquid at low pigment concentrations. However, at higher pigment concentrations (yet significantly lower than the maximum packing fraction) the dispersions behave like a cross-linking polymer at its gel point.

Rheological behavior of solutions of amphiphilic acrylic copolymers in mixed solvents

A descriptive model was developed, using viscometry and light scattering, that explains the anomalous rheological behavior of solutions of amphiphilic acrylic copolymers upon the addition of water. The rheological behavior can be explained by considering the relative magnitudes of three interactions : the intra- and intermolecular electrostatic interactions between the ionizable acid groups in the copolymer, the intramolecular hydrophobic interactions, and the intermolecular hydrophobic interactions. The initial addition of water enhances the ionization of the acid groups, causing the electrostatic interactions between the acid groups to dominate the other two interactions. This leads to expansion of the polymer molecules and, consequently, to a relatively constant viscosity during dilution with water. Upon attaining the maximum ionization of the acid groups on the chain, the intramolecular hydrophobic interactions dominate the electrostatic repulsion, and the chains start to contract. Further addition of water leads to aggregation of the polymer chains into large polymolecular domains, resulting in a sharp decrease in the viscosity. Intermolecular hydrophobic interactions dominate the rheological behavior in this stage of water dilution.

Rheology Control in High Solids Solvent Borne Coatings Containing Polymer Microgels

The focus of this work is to determine the type of interparticle forces that are primarily responsible for the rheology control of high solids solvent borne coatings containing polymer microgels. Rheological behavior is an important consideration when formulating high solids coatings due to the relatively low viscosities required for sufficient atomization during spray application and the difficulty of preventing the film from sagging during the bake cycle. Rheology control agents such as microgels are used to modify rheological properties where solvent evaporation is insufficient during the bake cycle to prevent sagging. In this paper that describes the first phase of our work, we have characterized several microgel dispersions used as rheology control agents in high solids automotive topcoats using scanning electron microscopy (SEM) and steady shear viscosity measurements. The steady shear viscosity versus microgel volume fraction data was compared with the expected behavior of ideal hard sphere dispersions. One of the microgel dispersions showed very nearly ideal behavior while others deviated from the expected behavior at higher volume fractions. We have also investigated the steady shear behavior of increasing concentrations of microgel dispersions added to low molecular weight acrylic polymer, typical of resins used in high solids automotive coatings. These microgel dispersions induce a yield stress in the Newtonian resin that shows a strong dependence on microgel particle concentration.