Static Image Analysis of Toner Particles

Toners are colorants for electrostatic printing processes. Toner is a very fine powder consisting of 5 to 30 μm particles. Due to the very small particle size, the powder is flowable and behaves like a liquid. Toner is composed of synthetic resin, pigments, magnetizable metal oxides and various auxiliary substances.

Toners are mainly used in photocopying machines and laser printers to create a print image on paper. Laser toner cartridges for use in copiers and printers come in sets of cyan, magenta, yellow and black (CMYK), allowing a very large variety of colors to be generated by mixing (Fig. 1).

The demands placed on the toner are very high: On the one hand, it must meet the requirements with regard to the printing result, on the other hand, the toner should adhere to as many materials as possible, except for the device itself (fixing rollers). Furthermore, it must meet the technical requirements of the device, must not accept moisture and must remain consistent in its consistency until use.

To ensure these consistent material properties, precise characterization of particle size and particle shape with an according particle analyzer is crucial.

Sample Preparation

Black toner and yellow toner, both as powder and as suspension have been submitted for analysis (four samples altogether, Fig. 1, right). As preparation for the analysis the two suspensions, two drops each were placed on a slide and covered with a coverslip. For the undiluted samples, about 10% of the image area was covered by the particles, as a result of which many overlays were detected (Fig 3, left). The samples were therefore diluted until a covered area of 0.5 - 1% was reached. For the diluted samples, almost all particles are well separated from each other, the effect of the overlays can therefore be neglected (Fig. 3, right). The two powder-like toner samples have been dispersed on a glass slide with the M-Jet module (70 kPa pressure).

Table 1:
Typical measurement conditions for toner particle samples wet / dry at 20 x magnification.

Particle Size results: Overview

Table 2 shows the percentiles of the Q3 distribution at 10 %, 50% and 90 % (d10, d50, d90) for all three size definitions x_{c min}, x_area and x_{Fe max}
The results for the wet and dry measurement of each color are almost identical. It is therefore possible to measure toner in wet or dry mode. However, the concentration of the liquid must not be too high. Dry sample preparation with the M-Jet works equally well as a wet dispersion.

Table 2: 
d10, d50 and d90 of all four samples, 20 x magnification, size definition x _{c min}, x _area and x _{Fe max}

Comparison with Laser Diffraction

Laser diffraction is an established method to measure the particle size distribution in a range from below 100 nm up to 2 mm. It is a very versatile technique, however, the reported size is always based on a sphere model, no information about shape, length or width of the particles is available.

Resolution, sensitivity and accuracy are better for image analysis. Fig. 8 shows a comparison between CAMSIZER M1 x_{c min}, x_area and x_{Fe max} with a laser diffraction result. The Q3 curve from laser diffraction is between x_cmin and x_{Fe max} and wider than xarea. This is very typical for the correlation between laser and image analysis.

Figure 8: 
Graphs of dry black toner sample CAMSIZER M1, x _{c min} (red), x _area (green) and x _{Fe max} (blue) and laser diffraction (black *). The distribution obtained from laser diffraction is wider than the CAMSIZER M1 result. Laser diffraction calculates particle size based on a sphere model. The result is therefore between the width and length measurement of the CAMSIZER M1.

Particle Shape Analysis

Simultaneously with the size measurement, the CAMSIZER M1 detects particle shape. The result of the shape analysis can be presented as a cumulative distribution (Q3). Round, isometric or spherical particles will plot on the right side of the diagram, more elongated, angular or non-spherical particles plot on the left side. Fig. 9 and 10 show the aspect ratio and the roundness of a yellow and black toner sample as Q3 distribution. Shape analysis reveals that the yellow toner is more spherical than the black toner.

Figure 9: 
Shape graphs of the yellow liquid toner (red) and black liquid toner (blue). Shape parameter aspect ratio (b/l); X_{c min} divided by x_{Fe max.}

Figure 10: 
Shape graphs of yellow liquid toner (red) and black liquid toner (blue). Shape parameter roundness (RDNS_C).

Summary

Particle size analysis and particle shape analysis of all four toner samples was performed with the CAMSIZER M1 static image analyzer. The liquid samples were diluted (2-3 drops in 20 ml deionized water). The dry powders have been dispersed on an object slide with the M-Jet dispersion module. The median is between 5.21 μm and 6.31 μm and the yellow toner particles are slightly smaller than the black toner particles.

Agglomerates may be ignored from the measurement by using a shape filter based on the parameter convexity (particles conv < 0.97 ignored). The repeatability is excellent, as shown in Fig 5. The results can be confirmed by laser diffraction analysis (Fig. 8). Particle shape analysis reveals that the yellow toner particles are more compact, have a higher roundness and higher aspect ratio than the black toner particles. A magnification of 20 x is sufficient to characterize the toner particles. Measurement at 50 x magnification is possible but will lead to longer measurement time if the same number of particles is analysed.

Appendix: CAMSIZER M1 particle size and particle shape parameters

The diagrams below show the most frequently used particle size and shape definitions used in image analysis with CAMSIZER systems.

x_{c min}

"Width"

Best suited for sieve correlation

x_area

"Diameter of circle with same projection area"

x_{Fe max}

"Length"