I
B. S. GARRETT, W. C. PRENTISS, and J.
D. SCOTT
Research Laboratories, Rohm & Haas Co., 500 Richmond St., Philadelphia 37, Pa.
Factors Affecting the Leveling of Latex Paints
D c n m z the course of studies on latex paint formulations, it became apparent that a method of evaluating leveling by other than brushing tests was necessary in order to relate one set of experiments to another. I t was concluded, after a thorough physical evaluation of the leveling phenomenon, that : 1. Leveling proceeds as long as the driving forces of surface origin exceed the yield stress of the paint film.
2. The rate of leveling is determined by the instantaneous value of the paint film viscosity. 3. The viscosity and yield stress of all paints rise sharply as water is lost beyond a critical solids content. 4. Water loss by wicking into a porous substrate is probably the major factor in differentiating between paints as to rate of change of properties. 5. Both an initial yield stress and wicking rate must be determined, in order to correlate paint performance with measurable properties.
Initially it was assumed that the most important property determining leveling quality was the yield stress of the paint. Yield stress as used in this study means the value of shearing stress obtained by visual extrapolation (to 0 rate) of a graph of shearing stress us. rotational speed of a modified Stormer viscomewr under arbitrarily specified conditions. After an analysis of the problem revealed the profound effect on leveling of water loss of a paint into a poroiis substrate, an empirical test was devised to measure this property. The test employed was one in which the rate of separation and rise of the aqueous phase into a strip of filter paper was determined (henceforth referred to as the wicking rate). Having two measurable properties available, a full factorial experiment was designed in an effort to relate the yield stress and wicking rate to observer leveling ratings. I n the study was included the effect of thickener type, thickener concentration, pigment volunie concentration, and extender. The paints were applied to a moderately poroiis substrate and given visual ratings for flow out. I t was found that the leveling phenomenon can be reasonably expressed ' as functions of yield stress and wickirig rate by the following equations: L1 = ( Y and,
+ 1) W'."
L, = ( y + 1) & 2 6 w - 1 )
A statistical analysis showed that the calculated leveling values, L, arid L,, correlated well with visual ratings of flow out. Using these equations to analyze trends, further formulation variables were examined which extended the full factorial design. Conclusions
Latex emulsion just before fusion
I t was concluded that the method outlined is applicable for determining the effects of formulation variables on leveling in general, and on the specific factors controlling the leveling phenornenon, the yield stress, and wicking rate. VOL. 51, NO. 2
FEBRUARY 1959
117
