Toner Particle Size and Shape Analysis

New manufacturing techniques

Early toner manufacturing involved the pulverizing and sorting of black, charged graphite. However, toner technology has become gradually more sophisticated in order to meet the ever-increasing quality and technical demands of the industry. The mobility of the toner in the supply reservoir mechanism, the transferability performance to paper, and the property of peeling from the drum are all affected by toner particle size, shape and material properties. In addition, colorants (predominantly pigments), resins, electric charge control agents and releasing agents have been added to toners as blend components. Fluidizing agents, lubricants and electric charge control agents have also been applied to the exterior of the toner particles.

Toner production methods have also continued to develop. There are two fundamental technologies applied to toner production – the pulverization method and the polymerization method. The conventional pulverization method, where an ingot (or a film) as raw toner material is pulverized and sorted, is slowly being superceded by the polymerization method, which is capable of yielding toner particles closer to a spherical shape. The polymerization method, which is also referred to as the chemical toner method is a technique in which granulation is conducted by utilizing an aqueous medium.

Size and shape measurement

With the ability to produce toners with more precise shape and size distributions comes the need to characterize such materials. Size can be characterized using the circle equivalent diameter - which is defined as the diameter of a circle that has the same area as the projected particle image. With this diameter, various irregularly shaped particles can be evaluated on the basis of a single consistent measure. Shape is characterized using circularity - a parameter that compares the perimeter of the projected particle image with the circumference of the area-equivalent circle thus permitting a numerical representation of complex particle shapes. Figure 1 shows an example of how circularity is calculated.

Figure 1: Calculation of circularity parameter

A circularity of 0.95-0.96 is optimum. Toner particles with a lower than optimum circularity value, act as an abrasive, reducing the lifetime of printing mechanism components and producing a lower quality image. Equally toner particles with a higher than optimum circularity (i.e. perfect spheres with a circularity of 1.00) act as a lubricant and do not transfer to the print medium properly.

Optimized image quality depends to a great extent upon achieving both a narrow size distribution (centered around a mean diameter of 8um-10um) and a narrow shape distribution (centered around a mean circularity of 0.95-0.96). Figure 2 shows the effect of toner particle circularity on image quality.

Typical low quality toner - low circularity and heterogeneous		Typical high quality toner - high circularity and homogeneous

Figure 2: The effect of toner particle circularity on image quality