Particle Size Analysis of Inks Using Dynamic Light Scattering and Laser Diffraction

An ink is a liquid containing various pigments and/or dyes used for coloring a surface to produce an image, text, or design. Ink is a complex medium composed of solvents, pigments, dyes, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescers, and other materials. Particle size of the pigment particles can significantly affect the color strength, surface finish, and durability of the final product. Measuring the particle size of ink pigments was typically done using dynamic light scattering (DLS). Laser diffraction can also now be used to measure these pigments due to the state of the art enhancements in the LA-950.

Components of Inks

The components of inks serve many purposes. The carrier, colorants, and other additives are used to control flow and thickness of the ink and its appearance when dry. The colorant can be either a dye (solution) or a pigment (particulate). Since the color strength of the pigment depends on the exposed surface area, a smaller particle size generates a stronger color. Other ink qualities affected by the pigment particle size include saturation, brightness, opacity, and viscosity. The opacity of the ink tends to maximize at a mean particle size near 0.25 - 0.50 µm. Particle size also plays an important role with ink rheology since a reduction in particle size increases the suspension viscosity. Particle size distribution is another important physical parameter requiring measurement and control. In general a wide distribution reduces color strength and can be indicative of poor stability. Figure 1 shows a basic diagram of how particle size influences several ink physical properties.

Generalized relationship between ink particle size and other physical properties

Particle size

Figure 1. Generalized relationship between ink particle size and other physical properties

It is often desirable to reduce the pigment to the primary particle size in order to achieve optimum benefits. Monitoring the milling process to achieve the desired size requires frequent use of a particle size analyzer. It is therefore quite important to be able to measure the particle size of ink pigments.

Particle Size Analysis of Inks

Most ink pigments fall into the range of 50-200 nm. Several techniques are capable of measuring in this range including dynamic light scattering (DLS) and laser diffraction. DLS can measure particle sizes within the range of roughly 1 nm – 1 µm, so this technique has been used to measure ink pigments for many years. Using the HORIBA LB-550 DLS system for ink pigments has been documented and can be found performing these measurements in labs around the world.

Particle Size Analysis by Laser Diffraction

Due to enhancements in small particle sensitivity laser diffraction is now also being used to measure sub-micron ink pigments. The HORIBA LA-950 has proven to be able to measure real world samples with mean sizes as small as 30 nm, but has the ability to measure larger particles up to the millimeter range. This technique provides more flexibility to measure larger agglomerates/flocculates and be used for other applications due to its broad dynamic range.

LA-950 Laser Diffraction Particle Size Analyzer

Figure 2. LA-950 Laser Diffraction Particle Size Analyzer

Experimental

Ink Sample 1 was analyzed on both the LB-550 and LA-950 systems. The concentrated sample was diluted in an organic solvent using both instruments. The operating conditions are shown below.

Particle Size Analysis with the LB-550

  1. Add 1 drop of the sample in the organic solvent. Mix well.
  2. Add one drop of pre-diluted sample to cell.
  3. Mix with pipette in the cell.
  4. Set the sample holder temperature to 25ºC and wait until sample reaches this temperature.
  5. Take 3 consecutive readings and record results.

Particle Size Analysis with the LA-950

  1. Fill MiniFlow sampler with organic solvent.
  2. Circulate until system reaches equilibrium.
  3. Align and blank optics.
  4. Pre-disperse 2 drops of sample into 15 mL organic solvent in beaker.
  5. Transfer sample from beaker into MiniFlow sampler using disposable pipette.
  6. Measure sample 3 times, record results.

Results from the LB-550 Particle Size Analyzer

The results from the LB-550 measurements are shown in Figure 3. Note that the particle size information is reported as a volume distribution in order to compare with those results from the LA-950.

DLS results for Ink Sample 1

Median : 145.9(nm) Diameter on %
Mean : 148.1(nm) :(2)10.00 (%)-101.4(nm)
Mode : 157.8(nm) (9)90.00 (5)-196.5(nm)

Figure 3. DLS results for Ink Sample 1

Results from the LA-950 Particle Size Analyzer

The results from the LA-950 measurements are shown in Figure 4.

Diffraction results for Ink Sample 1

Median Size : 0.19342(μm)
Mean Size : 0.30765(μm)
Mode Size : 0.1611(μm)
Diameter on Cumulaive % : (1)10.00 (%)- 0.1049(μm)
: (2)50.00 (%)- 0.1934(μm)
: (3)90.00 (%)- 0.5174(μm)
Std.Dev. : 0.4290(μm)
CV : 139.4322(%)
Span : 2.1324

Figure 4. Diffraction results for Ink Sample 1

Comparison of Results Using Dynamic Light Scattering and Laser Diffraction

Although these results are not identical, the similarity confirms the ability of the LA-950 to operate in this size range. The modes of the distributions are reported as 157.8 nm using DLS and 161.1 nm using laser diffraction, indicating essentially the same peak to the frequency distribution. The differences in the other reported results are not surprising since the two techniques measure different physical properties. It is quite rare that different particle size techniques generate identical results.

Determination of Particle Size Analysis of a Water-based Ink Using the Horiba LA-950

Ink Sample 2 is a water-based ink called Color Wonder. This sample was measured on the LA-950 using the procedure described below:

  1. Fill LA-950 sample handler with DI water.
  2. Begin circulation, align and blank optics.
  3. Add drops of the sample directly into the sample handler reservoir until the desired concentration (%T) is achieved.
  4. Measure 3 times and report results.

The results from the LA-950 measurements are shown in Figure 5.

Diffraction results for Ink Sample 2

Median Size : 0.10583(μm)
Mean Size : 0.10585(μm)
Mode Size : 0.1065(μm)
Std.Dev : 0.0120(μm)
Diameter on Cumulaive % : (2)10.00 (%)- 0.0896(μm)
: (9)90.00 (%)- 0.1237(μm)

Figure 5. Diffraction results for Ink Sample 2

Particle Size Analysis of a Water-based Ink Using the Horiba LA-950

Both DLS and laser diffraction are acceptable techniques for measuring the particle size of ink pigments in the 50 – 200 nm size range. The results shown in Figures 4 and 5 are actually overlays of three repeated measurements. It is impossible to see differences in the results to due the extreme precision of these measurements. The LA-950 is the most sensitive laser diffraction analyzer - capable of measuring below 30 nm - and the most repeatable and reproducible. These features combined with the broad dynamic range offered by laser diffraction make the LA- 950 a uniquely capable instrument for ink pigments and other sub-micron dispersions.

This information has been sourced, reviewed and adapted from materials provided by HORIBA Particle Characterization.

For more information on this source, please visit HORIBA Particle Characterization.

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