Laser Diffraction Technology for Routine Particle Size Analysis

The existence of laser diffraction technology for routine particle size analysis across a wide range of industries can be attributed to these two factors:

• The need for particle size data
• The ease of use of the technology

This article explains the reason why so many manufacturers need to measure particle size and the reason behind laser diffraction being the method of choice.

The Importance of Particle Size

Different industries measure particle size for the same reasons. Even though cement manufacture and fuel atomization are two very different fields, both have requirements to control the particle size for manipulating the rate of chemical reactions taking place when their product is used – hydration and combustion respectively.

Properties Affected by Particle Size

Certain essential properties influenced or controlled by particle size are:

• Reaction Rate – The rate at which a chemical reaction takes place is mostly a function of the specific surface area of the particles involved, the amount of surface area per unit mass. The finer the particle population, the easier it is for reactants to reach and react with a particle.
• Dissolution rate – The impact of particle size on the dissolution rate closely mirrors its effect on reaction rate. When particle size is reduced, the physical impediments to dissolution are decreased, accelerating the process.
• Packing density – Both particle size and distribution effect the way particles pack together. Larger particles packing are less efficient than smaller ones, causing larger void spaces. It is essential to control particle packing for producing ceramic and methal components using mould filling processes.
• Stability – Stability, in the context of a suspension implies avoiding sedimentation. A stable emulsion is one where droplets remain discrete rather than coalescing to form a continuous immiscible phase.
• Ease of inhalation – Human respiratory systems are highly effective in filtering out particles above a certain size to ensure the integrity of air supply and avoid irritation to the lungs. Particle size thus needs to be measured in connection with ease of inhalation due to the following reasons
• To prevent a product from being inhaled
• To ensure successful deposition of a drug within the pulmonary region
• Optical properties – The manner in which a particle scatters light is based on size. This phenomenon is exploited by coating, paint and pigment manufacturers to achieve desirable product performance.
• Consumer perception – With consumer products, food being the key example, of which are very essential; for example, coffee and chocolate.

Ceramic Industry

The particle size and distribution of ceramic powders is critical as it impacts the following:

• Amount of de-flocculant needed for successful casting
• Rheological properties of the suspension such as fluidity
• Properties of the finished product, modulus of rupture and fired color for example
• Thickness of the casting
• Drying properties of the casting – in case particle size is too fine then cracking is more likely

The Kreiger-Doherty equation describes the correlation between suspension rheology and particle size distribution at low shear rates:

where η is the viscosity of the suspension as a whole, η_medium is the viscosity of the base liquid, Φ is the volume fraction of solids in the suspension, Φ_m is the maximum packing faction of solids in the suspension and [η] is the intrinsic viscosity (2.5 for rigid spheres).

Figure 1. The impact of particle size distribution on suspension rheology.

Dairy Products

In the dairy industry milk is an excellent example of naturally occurring emulsion, while ice-cream and cream liqueurs are just two of the large number of processed products for which emulsification is an important manufacturing step.

The particle size of fat droplets impact the following:

• Flavor release
• Mouth feel
• Emulsion stability (the tendency of the product to 'cream' for example)
• Color
• Structural characteristics, such as the ability to hold air

Figure 2 shows data related to emulsion stability for a cream liqueur.

Figure 2. Variation in the particle size observed during the storage of cream liqueurs.

Introducing Laser Diffraction

Laser diffraction is a particle sizing technique, which means it generates a result for the whole sample instead of building up a size distribution from measurements of individual particles. Laser diffraction analyzers record the angular dependence of the intensity of light scattered by a sample, using an array of detectors.

ISO13320 offers a useful summary analysis of the relative merits of the two optical set-ups that now dominate commercial laser diffraction system design.

• The classic Forward Fourier setup highly common in previously developed instruments has the data collection lens positioned after the measurement zone as shown in Figure 3. This offers a wide working range and is therefore especially suitable for spray measurement, where the particles may be distributed across a wide path length. The lens here can be changed to focus scattering from specific angular ranges onto the detector array, thus enabling the measurement of different particle size ranges.
• In a Reverse Fourier set-up, which ISO13320 now recognizes as a standard alternative in laser diffraction instrument design, the lens is positioned before the measurement zone. The path length over which measurements are made is restricted by this setup but detection of scattered light is allowed over a wider range of angles, as detectors can be positioned both in front of and behind the cell.

Figure 3. Fourier and Reverse Fourier Lens Arrangements for Laser Diffraction Systems.

Pigments

The pigment industry depends on particle size data to control the optical properties of its products. Figure 4 shows the particle size data for calcium carbonate a filler used to whiten paper. For this product, within a narrow particle size band there is an increase in optical scattering efficiency that delivers optimal whiteness.

Figure 4. Measurement of calcium carbonate using Mie and Fraunhofer.

Automation

Complete automation of analytical technique greatly eases its transition into new markets and new application areas. Measurement with the best laser diffraction systems is automated by using standard operating procedures (SOPs). Once setup these comprehensive step-by-step instructions drive the analytical process from receipt of the sample through to data delivery, with negligible input from the operator. The only manual task may be sample loading The success of automation has underpinned the transition of laser diffraction into the process arena.

The Future

Laser diffraction systems are presently perceived as productive, flexible workhorses - a reliable part of a company's analytical armory. However, there are challenges remaining as industry evolves.

One main issue for laser diffraction instrument manufacturers is to make sure the performance gains over the last decade are fully exploited by the users.

Even though this article is focused on particle size, the parameter measured by laser diffraction, some sectors already know that the performance of their particles is a function of, size and shape. The combination of morphological imaging with chemical identification techniques, such as Raman spectroscopy, further increases the information flow.

Laser diffraction is complementary to, and complemented by, these newer techniques. It is highly effective to exploit them in combination. The benefits of laser diffraction suggest that its appeal will endure well into the future, and that the technique will retain its place as the preferred choice for industrial particle size measurement.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.