Process Design and Optimization of External Additive Blending on to Toner Surface

摘要

Additive blending is one of the most important steps in the toner manufacturing process. As toner sizes get smaller and the toner shapes and surfaces get more controlled, the step of blending plays a much greater role in imparting flow and charging functionality to the toner. For a given toner and additive formulation, the blending process significantly impacts the charge level, charge rate and the powder flow which are critical to xerographic performance. A functional analysis of the blending process is of considerable use in order to better understand the critical parameters involved and how they are related to functional performance of resultant powder in xerographic machine. A methodology for functional analysis and identification of critical parameter is presented for achieving good blend quality. The good blend quality is determined by toner/additives mixture harmony, uniform dispersion and distribution of additives and the optimal attachment of additives. Methods for the characterization of blend quality are presented. The additive attachment functionality is represented by the strength of the attachment on to the toner surface. The "Weak" symbolizes under-attachment (i.e., loosely attached or free additives), "Medium" represents functional level of attachment (optimal attachment strength) while "Strong" represents over-attachment i.e., additives are buried into toner surface and are non-functional for flow. This methodology takes into account the kinetics of additive attachment, dynamics of blend process and the heat transfer involved. Based on these understanding, we have proposed a set of process critical parameters that takes into account the kinetics, dynamics and the heat transfer. This methodology is demonstrated to be successful for process scale-up. The paper also talks about various proprietary blend tools and the dynamics imparted by these tools. To understand the dynamics of the blending process, it is important to quantify the forces acting on the batch to be blended. The blenders commonly used for toner additive blending are fluidizing mixers where the high rotational speed of the mixing tools fluidizes the batch of material. This allows all of the particles, regardless of particle size, density, coefficient of friction, and other characteristics, to intermix and disperse very quickly as low viscosity liquids. Hence it is plausible to consider aerated powder in a blender as a pseudo homogeneous phase. Thus, the theory applied to describe the dynamics of liquid mixing can be applied to solids blending by treating the air-solid mixture as a pseudo-homogeneous phase in a high speed mixer. To apply the liquid mixing theory to solids blending, we must be able to define the pseudo-homogeneous physical properties of aerated powder in the blender, and ignore, for the moment, the consequences of compressibility and inertially-driven stratification. A single phase computational fluid dynamic (CFD) model is presented for understanding the flow pattern generated by different tools. Several metrics have been proposed to evaluate these tools by performing parametric design using CFD.