Rheological behavior of a polymerically stabilized latex for use in water‐borne coatings

Abstract

A urtique urethane llnkage that permits chemical grafting of poly(ethy1ene oxide) (PEO) linear chains to the surfaces of polystyrene (PS) latex particles has been developed. Chemically grafting the functionalized hydrophilic PEO macromers to the PS particle surface allows the latex to be polymerically stabilized in a water-based medium. Advantages of the urethane linkage include the high yield of the macroiner synthesis and the hydrolytic stability of the final latex. Rheological experiments are used to examine both processing behavior and interparticle interactions fixlatex systems with different amounts of grafted PEO. Dynamic rheological experiments reveal that, at high macromer concentrations, the grafted PEO layer is effecti\,e in shielding the attractive interactions of the core PS particles that lead to flocculation. However, at low macromer concentrations, strong interactions are seen even at low particle weight fractions, indicating the presence of a flocculated system. Steady shear rheological evaluations show that the latex systems possessed suitable flow behavior for coating applications, even at relatively high particle weight fractions. Experimental steady shear data is utilized in conjunction with the Krieger-Dougherty equation to determine the size of the PEO stabilizing layer. The stabilizing layer thickness decreases as particle concentration increases, indicating a compressible system. Finally, the relationship between the strength of interparticle interactions and PEO graft density is gauged from the dependence of the power-law exponent of the elastic modulus on particle concentration.

Dispersion Polymerization of Polystyrene Latex Stabilized with Novel Grafted Poly(ethylene glycol) Macromers in 1-Propanol/Water

“…Numerous studies have synthesized and characterized polymer latexes with a stabilizing layer and have termed the latexes as sterically stable without providing sufficient evidence to support claims of stability. − , In the case of the present latex systems, we have employed steady shear rheological measurements to determine stability by analyzing the effect of stabilizer concentration on steady shear viscosity. (Additional steady shear and dynamic rheological analyses of these latexes are detailed elsewhere. , ) The steady shear rate (γ̇) is measured as a function of shear stress ( τ ) and the calculated steady shear viscosity ( η ), and the dependence of η on τ is shown in Figure for two latexes (LD5-003 and LD5-006) at the same particle weight fraction (0.38) but with different macromer concentrations. Particle weight fractions are determined by gravimetric analysis of the latex before and after drying.…”

“…The shorter-ranged steric interactions of our system enable more latex particles to pack into the same dispersion volume while maintaining a low viscosity relative to the electrostatically stabilized system. Such high-solids, low-viscosity sterically stabilized systems have strong potential for coating applications where a minimal water content is desired 10 Comparison of the high shear plateau viscosity (η ∝ ) as a function of particle weight fraction (φ w ) of a polymerically stabilized latex system with that of an electrostatically stabilized latex system. …”

“…(Additional steady shear and dynamic rheological analyses of these latexes are detailed elsewhere. 38,39 ) The steady shear rate (γ ˘) is measured as a function of shear stress (τ) and the calculated steady shear viscosity (η), and the dependence of η on τ is shown in Figure 9 for two latexes (LD5-003 and LD5-006) at the same particle weight fraction (0.38) but with different macromer concentrations. Particle weight fractions are determined by gravimetric analysis of the latex before and after drying.…”