Thixotropy and Physical Aging in Acrylic Emulsion Paint

In this article we study thixotropic and time-dependent rheological behavior of commercial acrylic emulsion paint using a framework of soft glassy rheology. We observe that acrylic emulsion paint, which shows thixotropic rheological behavior, demonstrates various characteristic features of a soft glassy materials such as weak but certain increase in elastic modulus as a function of aging time, maximum in viscous modulus as a function of strain at the onset of yielding, and slow increase in elastic modulus as a function of frequency. Stress relaxation measurements, subsequent to step strain, carried out at different aging times demonstrate slowing down of the relaxation dynamics. This behavior is similar to that observed in polymeric glasses and other soft glassy materials undergoing aging dynamics. Relaxation modulus when plotted against appropriately rescaled time yields a superposition irrespective of the aging time. The rescaled time also known as effective time, which adjusts the material clock by accounting for increasing relaxation time, enables estimation of the rate of change of relaxation time with the aging time. We conclude by relating thixotropy in an emulsion paint sample to an interplay between physical aging and shear melting in the same. POLYM. ENG. SCI., 51:2085-2092, 2011. (C) 2011 Society of Plastics Engineers